Rotten Creatures is a cool mod that adds high-quality entities, mainly zombie variants to specific biomes. In Desert you can find mummies, in the Nether you will find burnt zombies, and in the Tundra you will find Eskimo varients.
The mod is only in beta at the moment, so it will be interesting to see where the developer of the mod brings the mod. It has recently gotten Mod Of The Week on MCreator's website, and it is surely worthy to make the list for All Time Mods.
I recommend checking this mod out and exploring some of the biomes with this mod installed. The developer has a great eye for detail when it comes to modeling the entities.