Components V, S, M (a piece of the creature whose form you plan to assume)
This is one of the major limitations of this ability. Even if you have some way of casting this spell without material components, most DMs will likely require your character to have first hand knowledge of the race you're trying to emulate (IOW have actually met a member of said race).
There is, to my knowledge, only one way in the game to turn into something that doesn't exist and it isn't alter self. Alter self allows you to turn into humanoids, and the multiverse, while huge, is populated by outsiders. Aliens within the prime material are frequently monstrous humanoids, so you're pretty limited there as well.
I personally allow players to turn into fictional races, but they are limited to fictional races that exist in the books rather than only those that exist in world.
"A polymorph spell transforms your physical body to take on the shape of another creature." CRB pg. 211
A non-existing race doesn't have any creatures. The spell has to imitate something!
Mostly true, with the exception of Socothbenoth's 3rd tier evangelist obedience ability.
The demon lord of sexual perversion, granting the ability to add body parts. Yeah, I think I know where this is going. *cough*
The "centaur-like creature made only of human body parts" needs be no more specific than that.