Minecraft wiki entry for Spawn, associated text: In Creative or via a dispenser, the player can use spawn eggs to spawn most mobs. When mobs are spawned this way, all normal spawning requirements, such as light level and block type, are ignored (though monsters other than vindicators‌[BE only], evokers‌[BE only], shulkers, and ender dragon still cannot be spawned in peaceful).
Any entity can be spawned using the /summon command.