Again could be a clone, but… Old Man is definitely Myau!
First of all, I'm not sure that musk cat can live for 2000 years.
There is no clue if a Musk cat can leave 3 or 3000 years, but they call him OLD man for some reason. Even if a musk cat cannot leave so long there are things that could have caused her to be alive: consequences of eating a Laerma Nuts or have been cryogenic like Arisa (in Gaiden) and Lutz (in PS2). I believe he has more than 2000 years because of his lack of memory.
It's really strange... I have a feeling that this is not the first time we've met
Apperently he is talking with Lutz - the 5th or with Rika (the Numan = Human+Muskcat) or even with the game player (we). Anyway it is a strong sentence that makes me believe he is Myau.
This is a treasure that has been passed on generation after generation.
What about this information retrieved from Phantasy Star Drama CD (Sealed Memories)?
«Roy knows where the dome is, but for a price. In exchange for
her [Nei Third's] left-hand claw and the ring around her neck, Roy gives her a map to the dome and as "change"
he gives her the Nei Claw. The Nei Claw is an ancient relic which has been passed down his family for generations, datin gback to a time when a blue-haired young man came to the town to save it from monsters.»
This is talking about two claws: The nei claw and the left-handed claw nei3rd is wearing (I will call "Nei3rd claw" for short). I believe Nei claw is actualy silver fang or silver tusk, but, in any case, after Nei3rd death Lutz could have gave the silver tusk back to Myau for it/her to keep until the arrive of the choosen protector (Rika).
The hang up is definitely the Elsydeon scene, since that indicates he's dead, right?
Well, have you visited Myau's cave after receiving Elsydon? You'll find out that he died after giving the silver tusk.... ok, just kidding.
I don't believe Elsydeon had shown everyone witch died protecting Algol but every "Protector" (owner of that title) that scarified (not in the literal sense) him/herself to protect Algol -> everyone who fulfilled the mission he/she was meant to.
I believe the only ones who didn't appear were the "protectors" who didn't fulfill their mission yet. Myau did its job in PS1 and Lutz did the same (that's why he didn’t fight along with Eusis in PS2 and didn't appear in Elsydeon's PS2 scenes). Alys has fulfilled her mission too (fight DarkForce, Zio and special by protection the chosen one (Rudy) - she's dead right now so there wasn't more she could do to protect Algol ... her mission as a protector is complete).
Anyway I was completely satisfied with the answer that Elsydeon vision was only a resume of battle-history of Algol until this critical moment. It is a powerful conversation between the 1st hero (Arisa) and the last (Rudy), which gave him strength, especially on an occasion of doubt, witch he didn't know very well what to do at that time.