Moo Moo

Cows are a species of animals on Mushroom World and also the real world who are responsible for the production of milk. They have made semi-recurring appearances throughout the Mario series, usually appearing with different roles depending on the game they are in.

Mario and the Beanstalk
In The Super Mario Bros. Super Show! episode "Mario and the Beanstalk", Mario and Luigi had to get Gold Coins for the Mushroom Kingdom Orphanage, so they decided to sell Princess Toadstool's pet cow (the Royal Cow) to Dealin' Delbert. Delbert wanted to trade one of his many cows with Royal Cow, but the Mario Bros. wanted money. They instead traded Toadstool's Royal Cow for Garbanzo beans.

Koopenstein
Several Cows, with holes riddled throughout their bodies, later appeared as background characters in the episode "Koopenstein", where they inhabited the meadows of the Swiss Cheese Alps.

Mario Kart series
Cows can be seen in the background of a Mario Kart 64 and Mario Kart DS course named Moo Moo Farm. In Mario Kart: Double Dash!! ' s Mushroom City stage, a truck advertising this Moo Moo Farm course features a cow. Mario Kart Wii returns cows to the series, and this time, cows serve as obstacles in a course called Moo Moo Meadows. A truck in the Moonview Highway course, also has a picture of a cow printed on it with the text "Moo Moo Dairy".

Paper Mario
Cows are briefly mentioned in Paper Mario, as in the Chapter 4 Interlude, when Peach goes up to the milk, it says "Milk, fresh from the Cow!".

Mario & Luigi: Partners in Time
Kamek mentioned a cow in Mario & Luigi: Partners in Time; on Yoshi's Island before his battle, Baby Bowser was eating too many Yoshi Cookies very fast and ended up choking, whilst viewing the child choke, Kamek suggested he drink some milk from an "evil cow".

Mario Strikers Charged Football
Cows also appear in Mario Strikers Charged Football as course elements in Thunder Island; these cows become dangerous objects that fall down from a cyclone and raze anybody throughout the field.

WarioWare series
Cows have also made various appearances in the WarioWare Inc. series.

Trivia

 * Throughout the Mario series numerous cow-like creatures and characters have appeared, such as: Beefne, Red-Brief J, Bullies and Debulls, however any relation they may have to actual cattle is most likely very slim.