Ogres: Long before Gaiman did it in the new Beowulf, I decided that ogres were not a distinct race but rather the offspring of nobles whop had been seduced by hags. Great, dumb, foul creatures witha taste for human flesh, indeed, but by the laws that govern nobles, they cannot simply be exterminted. More than once it has led to a very interesting adventure when the PCs are hired by one person (the Lady, for example) to go destroy an ogre in the moors, but find themselves at odds with the Lord's men while doing so (who, of course, will not explain their actions). All the while, the hag watches and smiles, for her goal is to create chaos and cause havok and perhaps even find a way to legitimately place her monstrous bastard in the seat of power.
Centaurs: The Valoran Riders were the greatest heavy calvary mercenary company in the world an age ago. They fought for whichever crown offered the greatest portion of its treasury, but they were still skilled and honorable knights errant. However, they treated their mounts as the tools of war they were, and this angered Elhona, goddess of horses, and her druidic followers. After a time, the Riders and the druids went to war and the cruelty with which they used their mounts became too much for the horse goddess. Incensed, she cursed them, each man melded with his mount to become one being. She had expected wailing and rage, but what she found was a quiet weeping among war hardened men. "We do not fear death," they said to her, "we fear that out line is forever lost, for each one of us that dies can not be replaced." Taking pity, Elhona similarly "cursed" the many maids and hangers on that follwed the Riders wherever they went to war, giving the men wives so that the newly created centaur race would live on. In the centuries hence, the centaurs have become chivalric culture, knights never unhorsed, still waging wars with honor, but now always in defense of their goddess.