A zombie mob skin.
In the Overworld, zombies spawn in groups of 4 at a light level of 7 or less. Zombies have a 5% chance to spawn as a zombie villager and a 5% chance to spawn as a baby zombie. Baby zombies have an additional 5% chance of spawning as a chicken jockey. This also applies to spawn eggs.
Main article: Zombie siege
If a player is in a village with at least 10 doors and 20 villagers at midnight, up to 20 zombies may be spawned near the edge of the village regardless of light level or transparent blocks. It can happen in any biome, even if it is Mushroom Island.
Zombies can spawn from monster spawners inside dungeons.