I have some expansion ideas for the previous post. I'm figuring there would be a couple different reincarnation points, each with a different feel. There might be a legendary "Well of Souls" which acts like the standard Reincarnation spell, completely random. It would be in a dungeon filled with some of the dangerous random re-embodiments spawned by the well. In this location, slain animals will reincarnate as a random monster, rolling on the standard wandering monster table.
Other places are hazardous regions which reincarnate mortals who die there as specific varieties of creature. A chasm through a wild forest region might reincarnate people as elves, fey creatures, or intelligent forest animals; a mountain glacier reincarnates people as giants or eagles. Such places can cause mass reincarnations of multiple victims, slain simultaneously, as multiple individuals of a shared type, accounting for such things as a band of elves. It also suggests a rather horrific option for fantastic sentients to reproduce: lure groups of victims into a trap to be slaughtered en masse.
There could also be a cursed Valley of Monsters. Animals slain there have a chance of being reincarnated into a monster that melds one or more characteristics of each animal slain. This would be the source of things like griffons: they aren't the sexual product of a joining of lion and eagle, but are actually the necromantic union of the two. PCs attempting to exploit this would discover there's a 1 in 6 chance of something being incarnated at all, and a further 1 in 6 chance of getting the monstrous hybrid desired, but only if the slain creatures have total HD equal to the target creature. There's also a 1 in 6 chance something else will be incarnated, possibly an incomplete incarnation -- in other words, undead monstrous hybrid.