Zombie

Zombie is the Overworld's default threat: a slow melee undead that spawns anywhere dark, walks straight at the nearest player or villager, and burns in daylight unless it stands in water or shade or wears a helmet it picked up. Hurt one and it can call for help — a damaged Zombie sometimes summons others from the surrounding blocks, which is how one careless swing in a cave becomes a crowd. On Hard difficulty they break wooden doors, so a wooden door is not a closed shelter.

Their most consequential trick is conversion. A Zombie that kills a villager can leave a zombie villager standing instead, and on Hard difficulty that is guaranteed. Reversing it pays: throw a splash potion of Weakness, feed the infected villager a golden apple, and when it recovers its prices stay permanently reduced. Players infect and then cure a zombie villager on purpose for exactly that discount.

Water converts them too. A Zombie left submerged becomes a drowned, which is why flooded caves fill with them — but a converted drowned never carries a trident, so trident hunting means finding drowned that spawned that way.

A dungeon monster spawner is the dependable base for a zombie spawner farm, feeding rotten flesh to a cleric and levels to you. Two rarer outcomes come from elsewhere: a head drops only when a charged creeper's explosion kills one, and a baby Zombie can spawn riding a chicken as a chicken jockey, faster than an adult and able to follow you through gaps an adult cannot.