The Mischief Maker

Source
This item is mailed to the character by Minigob Manabonk after being "Manabonked!" in Dalaran (You can get manabonked both in the Underbelly and in the surface town).

It will, one time, turn a character into a sheep, with the following limitations:
 * doesn't work on shapeshifted druids (who are immune to polymorph when shapeshifted)
 * only operates in Dalaran