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 not available in Item Selection for certain reasons. The player may spawn their chosen mob by right clicking whilst holding the egg, aiming it at an open space.

Available Mobs
Spawner eggs are also obtainable by using a SMP command give. Contents of the egg is determined by the damage-value parameter, it is the Entity ID of an object that will be spawned. Eggs created this way can spawn mobs normally unavailable in Creative Mode (e.g. Giants), and entities like Minecarts, or even Primed TNT.

Trivia

 * Although not officially stated, one might determine the reasoning behind some mobs being unavailable. 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 by Spawn Mooshroom.
 * 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.
 * In 3rd Person view all Spawn eggs have the unaltered sprite in the players hand.