Sheep are a species of animal that are responsible for the production of wool. In the Mario series they have the ability to induce sleep and often appear in the Paper Mario series where this ability is put to plenty of use.
Sheep Rustle Hustle
Sheeps make their debut appearance in the Saturday Supercade episode "Sheep Rustle Hustle". In the episode a group of Sheep rustlers attempted to steal a herd of Sheep from the farm of a boy named Biff, but with the help of Donkey Kong Jr. and his friend Bones, they manage to save the Sheep and capture the rustlers.
Super Mario RPG: Legend of the Seven Stars
Sheep make their first appearance in a Mario game in Super Mario RPG: Legend of the Seven Stars. An item known as the Mystery Egg can be bought by Mario from the Treasure Hunting Toad, if used ten times the egg will hatch into a Lamb who will then say "You're a great shepherd!" and the Mystery Egg will become the item Lamb's Lure. A Sheep can be summoned with the use of Lamb's Lure and after being summoned the Sheep will turn an enemy into a Sheep, then both the Sheep and the transformed enemy will fly off the screen. The Lamb's Lure can also be upgraded into the item Sheep Attack, this item can summon an entire flock of Sheep which turn all the enemies on screen into Sheep which will then fly off. These items do not work on bosses or large enemies.
WarioWare: Smooth Moves
Mario Party Advance
In Mario Party Advance, a Gaddget known as Snooze Ewes appears which seems to be a type of music box that can put the player to sleep by playing soothing music and showing numerous Ewes jumping over a fence.