Over a hundred mobs are scattered across each biome and dimension, and each one actually varies from the other one! Along with facing off new creatures like the reptilian Makas and the shadow-based Grues, you can tame mounts like the Warg.
Alex's Mobs is a Forge mod that adds 89 new mobs to Minecraft. All of these mobs fall into either two categories: most of them are real world creatures, like Grizzly Bears, Roadrunners, Orcas, etc; but some of them are purely fictional, like the Endergade and Bone Serpent.