Beta Bred offspring.
Spawned from a bucket (e.g. Fish)
Spawned due to a chunk being generated.
Spawned via the /summon command.
Mob converted from another mob (e.g. Villager -> Zombie Villager).
Mob transferred from another dimension.
Spawned via dispenser.
Mobs spawned due to some in game event.
Jockey spawned with the mob they ride (e.g. Chicken jockey).
Mob loaded in from a saved state.
Mobs summoned via other mobs (e.g. Vex).
Natural Mob Spawning.
Mobs spawned due to a patrol.
Mobs spawned from the reinforcement attribute (e.g. Zombies).
Spawned from a spawn egg.
Spawner spawns.
Mobs generated specifically within some structure (Nether Fortress, Ocean Monument, Witch Hut, etc).
Mobs spawned from a Trial Spawner.
Some type of player triggered mob spawn (e.g. Skeleton Horse Traps, Wither, Snow Golem).
An enum containing reasons an entity could be spawned.