1ed (and maybe 2nd ed which I only played a couple of sessions) does not really scale that well for monster PC, unless as mentioned it is a short run or all PCs are in a similar position.
There is Savage Species for using monsters as PC, even if it is 3.0. I'd consider the centaur based on several things (fits with the theme/"logically" fits in under a back story, was on a par with the other PCs). But a monster, even "pet/prisoner" of another PC would face a lot of problems in many environments (civilized, humanoid lands, PC horses, natural predators/prey).
Reactions of others may be what Dandu means by shooting themselves in the foot.