Mystery Egg (Super Mario RPG: Legend of the Seven Stars)
From the Super Mario Wiki
Mystery Egg is an item from the game Super Mario RPG: Legend of the Seven Stars. It is obtained from the Treasure Hunting Toad in the Moleville item shop for 200 coins. It is initially useless, however, it can be "hatched" into a Lamb's Lure by using it 10 times with Toadstool while equipped with the B'Tub Ring. After the tenth time, a sheep will hatch from the egg and say "You're a great shepherd!". If used by someone other than Toadstool, it won't count and the item will buzz.