This is a question Ive been asking myself. They're kind of the combination of two (or more) creatures into one creature, except for a lot of taurs Ive seen around its the same kind of creature twice. Does that count as a chimera?
And it gets even more muddled when you look at the root origin for the terms too, since centaurs and the mythological chimera were both specific things and not at all the same thing, but language has evolved since then too...

