Minecraft wiki entry for Development_versions, associated text: Horse was split into entity IDs horse, donkey, mule, skeleton_horse and zombie_horse for their respective types.
Skeleton was split into entity IDs skeleton, stray and wither_skeleton.
Zombie was split into entity IDs zombie, zombie_villager and husk.
Guardian was split into entity IDs guardian and elder_guardian.
Unused savegame IDs Mob and Monster were removed, meaning that all references to the first mob added to the game called Human, have been removed.