In Mario Kart Wii and Mario Kart 8
Moo Moos (also known simply as Cows) are cows that lazily roam around the Moo Moo Meadows stage in Mario Kart Wii. If you run into one, you will spin away. There are many of them in this level. Moo Moo Meadows appears in Mario Kart 8 as the first course in the Shell Cup. In Toad's Turnpike, the fourth and last course of the Shell Cup, a truck is styled and named "Moo Moo Meadows Milk" The word "milk" at the center and larger than the others.
In Mario Kart 64 and Mario Kart DS . . .
They also appear in Mario Kart 64, in the level Moo Moo Farm. These ones do not go on the track at all, instead staying behind the fence. They actually have no animation, just face-me models that look like cows. However, they can be heard mooing.
The music for this level is also used in Yoshi Valley. Moo Moo Farm also appears in Mario Kart DS as the second race in the Shell Cup. Incidentally, this also appears as the second race in the Mushroom Cup for Mario Kart 64, the game this track originated from.
In GCN Mushroom Bridge, a truck is style "Moo Moo Dairy". It looks like it is delivering dairy to Coconut Mall.
In Mario Kart 7 . . .
Mario Kart 7 also shows a relationship with Moo Moos. In Coconut Mall, a shop has the title "Moo Moo Dairy". It is closed, so you cannot go through it. The only shop that you can go through is Delfino Dream, a clothing store.
In Mario Kart: Double Dash . . .
In Mushroom Bridge, a truck is styled "Moo Moo Dairy". This happens in Mario Kart DS.