I'm really like the idea of a powerful masculine centaur willingly being treated as a mount by a beautiful she-male or pretty boy owner (we can discuss which you prefer). I really like the BDSM aspect of this. The DM may rule that you are a cohort of mine rather then another pc. My character (and likly the centaur) are evil.
It's for this game http://elliquiy.com/forums/index.php?topic=169678.0
EDIT: I chose not to write a huge backstory since the setting is described in the above link and our relationship is something to be discussed and created together.