Whenever a passive adult mob is right clicked while holding a spawn egg of the same animal type, it will produce a baby version for any mob that has one, excluding Squids and Bats .
In Minecraft 1.4.4, whenever a baby version of a Villager is spawned, it would always produce a farmer villager child. This was later fixed.
Baby Wolves will spawn sitting if spawned using spawn eggs will produce pups that are sitting by default. This can cause a glitch to where the pups move while sitting if the player exits and re-enters the game.
As of 1.5 the player can rename eggs, and when killed by that Mob, the game would say " [Player] was Killed by [Name of Mob] " (E.g. Notch was Killed by Herobrine).
In 1.6.1, a discoverable coded feature was added: If a Mob's Spawn Egg is renamed with an Anvil or Name Tag , and is called "Dinnerbone" or "Grumm", it will be flipped upside-down. Any upside-down mobs will have right-side-up effects on them (e.g. a Zombie named Dinnerbone burning will be upside down, but the fire on it will be right-side-up).
If the player uses a Skeleton Spawn Egg in The Nether , it has a 80% chance of spawning as a Wither Skeleton.
Naming a sheep spawn egg "jeb_" will cause the Sheep's wool to be rainbow colored in 1.7.
A Donkey and Mule spawn egg are available in the Console Editions of Minecraft as an exclusive feature.