When you get a disguise that you don't already have, there should be a notification that says something like this: "Congratulations! You have received a (insert mob type here) disguise. Do /d (mob name) in order to "disguise" yourself, and appear as a/an (mob name here) to other players. Do /ud if you wish to undisguise yourself." This will benefit many new players, as many players receive a disguise from a crate, and end up asking in chat why the crate "gave" them an egg, but they see nothing new in their inventory.