Sheep

Sheep are a species of animal that have appeared on both Mushroom World and Earth 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. Males are known as Rams while females are referred to as Ewes.

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.

Paper Mario
In Paper Mario, an item known as Sleepy Sheep appears in the game which can be used to summon a large flock of Sheep which will put most enemies on the field to sleep. The item does not work on bosses though.

Paper Mario: The Thousand-Year Door
In Paper Mario: The Thousand-Year Door, the item Sleepy Sheep appears again with the same effect as in the previous game. Sleepy Sheep can also be made by combining a Point Swap and a Tasty Tonic in Zess T.'s kitchen.

Super Paper Mario
In Super Paper Mario, the item Sleepy Sheep returns once again where it works the same as in the previous two games. In Saffron's kitchen on Flipside, a Sleepy Sheep can be cooked into a Mistake which restores 1 HP.

WarioWare: Twisted!
In WarioWare: Twisted!, Sheep appear in the microgame Falling Asheep where the player's objective is to make Sheep jump over a fence by tapping the A button.

WarioWare: Smooth Moves
In WarioWare: Smooth Moves, Sheep appear in the microgame Shear Terror where the player must shave a Sheep as fast as they can.

Wario World
In Wario World, Rams appear as enemies in Beanstalk Way where they will leap and crash towards Wario with their horns. They can be easily stunned and defeated with a Wild Swing-Ding, Piledriver or a Power Throw.

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.

Super Smash Bros. Brawl
In Super Smash Bros. Brawl, a Sheep-like monster known as a Towtow appears as a common enemy throughout the game. Towtows at first appear as serene creatures blissfully sleeping, but upon being awoken they become red and vicious and proceed to attack anything in their way.

Mario & Luigi: Dream Team
The character Dr. Snoozemore resembles a sheep.

Trivia

 * In Mario & Luigi: Superstar Saga, a wearable item called Wool Trousers appears in the game, which may be made out of Sheeps' wool.