Spawn Egg

A Spawner Egg is an item added in the weekly Pre-release 11w49a, which is only accessible in Creative Mode. It allows the player to spawn a variety of mobs on command, although some mobs are excluded for certain reasons. The player may spawn their chosen mob by right clicking whilst holding the egg, aiming it at an open space.

Trivia

 * Although not officially stated, one might determine the reasoning behind some mobs being unspawnable. Snow Golems can already be created by the player. Spider Jockeys are two separate entities. Giants have been removed and are not supposed to exist. Enderdragons are a boss mob meant to only spawn in The End, and to spawn them in the Overworld would be dangerous to the landscape and any structures made.
 * There is only one texture for the Spawn Mob eggs. The game hue shifts this texture to create different types. The unaltered sprite is used to Spawn Mooshroom.
 * Attempting to obtain a Spawn Mob using the Give command in SMP will give the player the template egg called "Spawn," which has no effect when used.
 * Since they are not used as projectiles, attempting to fire them from a Dispenser will result as them simply being dropped as an item.
 * Mobs spawned via this item act the same as normally spawned ones. As such, hostile mobs will despawn if the player ventures too far.