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. 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.