Furby equivalents of many of the standard Minecraft hostile mobs will spawn in the world, including:
ZombieFurby
CreeperFurby
SkeletonFurby
EnderFurby
FurbyGhast
WitherFurby
You can summon the Furby version of the Wither by building a structure from digested redstone blocks and WitherFurby Skulls, in the following pattern:
WitherFurby Skulls are rare drops for SkeletonFurbies.