If they actually existed, and we call those that have the form of their phenotype in part or full a therian, then possibly.  It seems far more likely that centaurs were born from inaccurate recounts of men on horses than were therians.  The reason I think this is that centaurs do not fit into nature.  A were-form I believe would merely change the bone structure into something more animal-like, not actually create whole new muscles, limbs, and brain functions.  Perhaps I am mistaken, but as I am skeptical of "physical shifts" in the first place (especially those that add new body parts completely), I'd say that those creatures were born of imagination and faulty vision, much like unicorns and other mythical beasts.  I'd call them halfbreeds if anything, just like the half-animal gods of certain cultures, harpies, sphinxes, etc.  They are an odd *physical* combination of beast and man rather than a mental one.  There is only the half form; there is no change into the beast and no truely "human" part about them other than their physical shape

The minotaur is a bit more plausable than the centaur, but at best I'd say it could be a huge, bull-like man rather than the half-beast that legend dictates.  However, if these "were-creatures" really *did* exist, a minitaur could definitely make a case for being a were-bull, should it change back into the form of a man at some point.  I don't quite remember all the mythology there, unfortunately, so I can't tell you if that fits according to legend until I have time to go look it up again.  I remember the maze, but that's about it. 

Having done research into this actually, the thing about the Minotaur was that he was the son of a queen who had sex with a white Bull that Posiedon had expected to be sacrificed for himself, but the King (I forget his name, he's the one Daedalus escaped from only to lose his son) had decided to keep the bull, so the sea god and another god got together and made her lust for the bull, but anyway, there was only one, and it was kept in the labyrinth which Daedalus had designed, because the king did not want to kill the son of his queen (it was later killed by some warrior with the help of the princess who had fallen in love with him)  I don't think you could consider the Minotaur to be a were, as it was more of just an enraged beast, half human, yet without humanity, half bull yet without any sense of being a bull.  Then again, I, like firestar here, think it's just a myth based on something real (I believe the king was Tyranrus, who, if I remember right, is historically known as being so evil, he was killed by his own people in a greusome way that he himself had devised for his enemies: The Braizen Bull, a metal bull which you put people in and cooked alive), just as I'm pretty sure centaurs, which also wouldn't be considered weres because they had no true humanity nor were they horses, were just people who had never seen horse-riders before thought what they saw was one horrifying animal.
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