Ok, here's the theory.
Silver Fang
Before AW342 high purity Laconia was already forbidden to be forged,[026] sold or used[025] in weapons by Algol’s ruler (Reipard Lashiec).[001][002][003] Only non-laconian or low purity laconian (silver)[004][005] weapons could be bought on shops. This was the case of a simple “silver fang” Alisa bought to Myau.[061][006] The silver fang defeated Lashiec and Dark Force (in Palma).[007]
Animal Claw
In AW1284 musk cats have a treasure called "animal claw" that they would give to Nei if she was alive at that part of the game.[027]. Musk cats cannot craft or buy items [008][009][018][022] so the “animal claw” was a weapon gained in the past, that passed through generations of musk cats, making it an ancient weapon. The only weapon that was in possession of a Musk Cat, that we know, was Silver Fang.
If this is Silver Fang, then who possesses "Animal Claw" are Myau's descendents who live in Skure (Dezolis). However, if they are really Myau’s descendents, then Myau is not in Skure anymore. He could be dead or simply missing (what happened to him is a mystery), but the fact is that 3 years have passed since their independence from humans[029] and they still didn’t choose a chief,[028] an elderly master...
Eventually they, or their descendents, went to Mysterious Valley[030] since 1000 years latter no more musk cats lived Skure[035]. They must had the help of someone (Myau?, Lutz?, both?)[031][032][033][034] to find the "Mysterious Valley", since, in PSII they were in dangerous during 3 years and never left Skure.[029].
Nei Claw
In AW2281 an old man (in Chaz and Nei3rd perspective), named Roy, tells that, after PSII[036], a blue haired guy (Lutz or Rolf) entrusted a claw to his ancestor.[037] That ancestor lived near the Silver-Dome[039] (in Motavia) where Nei2nd was being studied by an AI called Gene (at least)[040][041].
Like the “animal claw” this one was intended to be given to Nei2nd[042] and, in fact, was later given to her (to Nei3rd, a direct clone from Nei2nd who possess Nei2nd memories)[043][044]. As we know by PSIV and PSIII, Espers (Sages/Telementals/Casts/Caseals/whatever) could baptize/bless/’put their feelings into’ weapons[045] in order to conceal them holy powers[059][049][002]. In some cases,[058] on that kind of ceremony they would give them the ancient word of power (“Nei”)[057] creating the "nei weapons"[054] (weapons became baptized as “Nei”[060][054]); in this case it was Lutz who create “Nei Claw”[050] in the same way Espers did in PSIII with Myun’s claw.
Coincidently, in all Phantasy Star games only legendary weapons (for example: weapons that faced DF)[060][058] became “Nei”, which could also apply to “silver fang”, since, like the legendary weapons from PSIII, was also used in the ancient battle against DF[007].
The Neiclaw was last heard to be in Seed (the bio-dome) -that was in AW2281-, but in AW2284 (PSIV) there's no trace of it there.[056]
Silver Tusk
Silver Tusk is in possession of a “master elder” musk cat (in Dezoris).[051][052] The “old man” says that it has been passed through several generations,[053] a characteristic shared by “animal claw” and “nei claw”. Like “animal claw” this weapon is also a treasure from Musk cats and like both “animal” and “nei” claws[042] it is intended to be held by a newman.[051][052][053] Another characteristic is the low purity of Laconia this claw is made,[004][005] like Myau’s ultimate weapon from PSI.[007] Like a nei weapon (any of them) it was made to fight darkness.[049][050] In short, this last claw gathers all the aspects from the previous ones.
5 things that prove(?) they are all the same:
1- The neiclaw was created by Lutz and was intended for Nei2nd.[042][050] In the case we resurrect her why didn't Lutz give the neiclaw to her like he did with Neisword to Eusis? (PSgen2) Because he didn't have any, that's why. The legendary weapon was with the musk cats at Skure... and she already possessed it.
2- An item can only be transformed into "nei" if the weapon was already legendary. The only legendary claw is the silver fang.[058]
3- Silver fang is the silver tusk[yeah!]
4- Very unlikely Musk cats' could have other fang as a treasure that wasn't from Myau's.[006]
5- The thing that most amazed Chaz about Neiclaw was the fact it was only a single hand claw.[021] This could be explain if we have in mind musk cats could only use 1 claw (on their mouth as a fang).[023][024]
Sources
[001] "Do you know what the hardest, strongest material in the world is? It’s ‘Laconia’! Laconian weapons are the strongest [in the solar system]" (PSI)
[002] "Weapons made from laconia conceal holy powers, y’know. I heard that LaShiec despises them, so he hid them here and there throughout the planets of Algol." (PSI)
[003] "What? ‘Overthrow LaShiec?’ If you really are going to do that, you must gather all of the Laconian weapons: Sword, Axe, Shield, and Armor. These weapons are more powerful than any others." (PSI)
[004] Item description from PSIV: Silver Rod "A rod made of low purity Laconia.
[005] "Do you know of a metal called 'Laconia?' With Laconia you can make weapons and armor that is out of this world! We haven't caught up with refining techniques here so the best we can do is make the low grade 'silver.'" (Palman in Dezolis, PSIV)
[006] For beeing a cat Myau has no hands and she/he cannot perform simple tasks like we do with our hands.[008][009]. Musk cats cannot open the top of the bottles[008][009][018] so Myau cannot also forge weapons. He cannot buy or trade things, not only because of his/her hands but also because cats are objects of trade.[010][011][012] He needs the help of humans, dezorian or motavian to live with humans (this includes buying things).
[007] In PSI there are only two weapons for Myau. The "Iron Fang" (Attack power +10) and "Silver Fang" (attack power +30) which is the weapon used to defeat Lashiec and DF (it is the last to be found and it has more (+20) attack power the other).
[008] "Someone turned him into stone. I can heal him with this medicine, but I can’t open the lid, so..."
[009] Use Alsulin: "Alas, Myau was not able to open the lid to the bottle."
[010] "Say... That’s a real unusual pot you got there... ...is 'dat a ‘Laconian Pot’?! How ‘bout I give you da animal, you give me the pot? Whuddayuhsay?"
[011] "When you're lonely, the best way to console yourself is to keep a pet!" (Publicity in a newspaper)
[012] Three years before PSII everyone could buy musk cats like it was done in PSI[011][010]. In PSIV Dezolis still uses pets as a business[014][015] but since Musk Cats are hidden in Mysterious Valley they turned to Dezo penguins and even started to domestic dezo owls[013]. In PSII Musk Cats were also a profit for other business like magic caps sellers[016] or its black market[017].
[013] "The Dezolis Penguin is really cute! We made improvements to the Dezo Owl species so that they can be bred in captivity." (PSIV, Dezolian at Zosa)
[014] "The egg of the Dezolis Penguin is our special product!" (PSIV, Dezolian at Zosa)
[015] "Wouldn't you like to walk along with the Dezolis Penguin?" (PSIV, Dezolian at Zosa)
[016] "Why don't you talk to your pet, too? Please come to our department store for a magic cap!" (Publicity in a newspaper)
[017] "Recently, there's been many fake magic caps going around. Please be careful." (Publicity in a newspaper)
[018] Only with much training a musk cat can open the top of a bottle,[019] Still, crafting weapons is impossible for them.[022] The only way to have weapons is to steal them from from defeated Skeletons in Parma,[020] that only exists once every millennium (with DF). Besides that he can only use a single claw... the silver fang is a solo claw like neiclaw.[021]
[019] "Meow. I can get the top off of this bottle, meow." (Musk cat with -possible- Myau's bottle, PSIV)
[020] "They [skeletons from Palma] are pretty easy. Just attack them normally and they should die, leaving an Iron Fang, a weapon for Myau." (Orakio's walkthrough)
[021] "But normally isn't a claw something you're supposed to wield in both hands? There's only one of them.. (Chaz, Phantasy Star IV: Sealed Memories)
[022] A tusk for cats is a claw for humans[024]: since musk cats cannot use their hands (the only record we have is that only a one musk cat could open the top of a bottle and even he was barging about that)[019]. They had to use their mouth in order to perform attacks[023] with those weapons, that's why they could only use a claw instead of two.[021]
[023] Although a US cover it expresses the same idea: http://gaygamer.net/images/ps-cover.jpg
[024] Rika can equip Silver Tusk with any other claw. She uses Silver Tusk like it is a claw. (PSIV equipment menu and in-game battle)
[025] "Do you know of a robot called ‘Hapsby’? It’s a robot made out of laconia! But it seems to have been dumped into a scrap heap!"
[026] "That laconian pot was worth a fortune! Thanks a bundle!" (PSI)
[027] http://www.youtube.com/watch?v=GVA7nLw_ ... re=related
[028] "I'm the chief of this valley. Hmm, there is good in your eyes. No wonder you were let through."
[029] "It's such a surprise to see a human other than the clone lab Grandma! Everybody left three years ago."
[030] "This is the mysterious valley, meow" (PSIV)
[031] "Somehow, we get a feeling of fond rememberance from you, meow." (PSIV)
[032] "Very seldom do humans enter this place, meow." (PSIV)
[033] They went to Mysterious Valley to never happen that again! Being made human pets and then abandoned by them. They refugee from humans and dezolians to be free and protected from danger.[028]
[034] In my opinion, Lutz turned "animal claw" (Myau's silver fang) into a Neiclaw after leading them into Myst Valley (they gave ito to him -or to Rolf- instead of Nei, since she was dead).
[035] "All that is left of the musk cats are here, meow." (PSIV)
[036] Lutz created neiclaw,[050] but he was in cryogenic slee from AW1274 until AW1284,[038] then the blue haired man (Lutz or Rolf) could only give the claw after PSII.
[037] "From a long, long time ago, back when monsters had come to this village. A blue-haired young man who had saved this village had entrusted the weapon to one of my ancestors for hiding. He said he would come back once more for it. But that young man never again came to this town."
[038] Lutz also waked up every ten years, however before AW1282 biomonsters didn't exist and when the blue haired man gave nei claw to Roy's ancestors they already existed,[037] so [036] remains true.
[039] "Mmn, the dome that you're searching for is probably 'that place'. It's about as far away as the village I live in (...) this mark here is this village. So, leave the town through the north gate and head towards the north east. The landmark will be this rocky mountain. If you make it there, you should be able to see the dome from it." (Roy, Phantasy Star IV: Sealed Memories)
[040] "You are NM-1153. Based on a sample of genetic information prior to the 'Great Collapse', I was able to complete you by cloning the damaged portions of DNA. You are the ultimate masterpiece of the NM-1000 Series." (Gene, Phantasy Star IV: Sealed Memories)
[041] "As a remaining system of the 'Great Collapse', it is my task to perform Neitype research. You were created as a step towards birthing a new race capable of surviving the harsh environments following the 'Great Collapse'."
[042] "This is called the "Nei Claw", and I've heard it was created for the person who bears its name." (Roy, Phantasy Star IV: Sealed Memories)
[043] "So in the end, I think this weapon is more suited to being in your custody than in mine. " (Roy, Phantasy Star IV: Sealed Memories)
[044] "According to calculations, this is not supposed to occur. Somehow, through cloning it seems that the subconscious part of the original specimen is making itself known. * In the form of dreams, it has been displaying itself to you in bits and pieces. "
[045] "In the past, a spell binder came to town and taught me to put my feelings in metal. And this is the store that inherited that knowledge."
[046] "Use the Nei weapons to destroy Dark Force."
[047] Low grade laconia (silver)[002] or high grade laconia[049] can conceal holy powers. Silver items are using magic (thunder, flaeli, hewn) to compensate[048]. However, every metal can be embeeded with power [045]
[048] Here http://www.rpgclassics.com/shrines/genesis/ps4/weapons.shtml's a table where you can figure what might be the weapons that are using the power of something. You'll conclude they must be the silver ones since they are beeing seld in Dezo and have less power attack than laconia.
[049] Item description: Laconian Rod "Rod made of hi-Laconia. Has power of 'Efess'."
[050] "It was the legendary magic user Lutz who created it" (Roy, Phantasy Star IV: Sealed Memories)
[051] "The old man has a treasure, meow." (Before receing Silver Tusk) "Please put the old man's treasure to good use, meow." (After receing Silver Tusk) (Rika is the only member able to equip and use this treasure)
[052] "We haven't even seen the old man's treasure, meow." (Before receing Silver Tusk) Please put the old man's treasure to good use, meow. (After receing Silver Tusk) (Rika is the only member able to equip and use this treasure)
[053] "This is a treasure that has been passed on generation after generation. Take it with you. It's the Silver Tusk!" (Old man to Chaz's group. Rika is the only member able to equip and use this treasure)
[054] "Use the Nei weapons to destroy Dark Force."
[055] See the forum treads: "Silver Tusk vs Guard Claw"
[056] In my opinion Lutz (Rune) returns to give that claw back to musk cats,[031] since Nei is dead and musk cats could choose the next protector to have it when he (Rune) brings he/she there. Also the blue haird man promised that would come for the neiclaw[037]... this was the only way to not break the promise.
[057] "Nei is the word of ultimate power."
[058] "The lost word cannot be revealed until all five legendary weapons are gathered together."
[059] "The ancient word of power is the word "Nei."
[060] "The ancient heroes used the legendary weapons. These are Orakio's Sword, Miun's Claw, Siren's Shot, Laya's Bow, and Lune's Slicer."
[061] "At the end is the village of Loar [in Motabia] (...) There is also a Silver Fang here (...) Buy Myau a Silver Fang"