The Mushroom Biomes are some of the rarest and most striking areas in the game. They are quite unlike any other biome in the game, and their lore is just as fitting. Basically, these biomes throw off parasitic spores that latch on to any mobs in the area and convert them into mushroom-hybrid mobs.

Evidence for this exists in the Mooshrooms that inhabit these zones. If shears are used on them, the mushrooms come off and the animal converts to a normal cow once more. This theory is strengthened by the fact that no mobs hostile or otherwise can spawn on Mushroom blocks. It seems even the evilest creatures keep a healthy distance from Mushroom Biomes.