One of the quests with the Winterfin Murlocs gives you the Blizzcon Murloc suit to wear. It allows you to sneak into a cave to complete the quest.
Unfortunately, it only works in a very defined area and you have to turn it in when you are done with the quest. I can't bear to give the suit up at this point, I've always wanted one, so I'm still carrying it around.
When you do the quest, make sure you try out the emotes while wearing the suit. Some of them are really fun!