Moo Moos are a species of cow appearing as obstacles in the Mario Kart series.
Mario Kart 64
Moo Moos make their first appearance in Mario Kart 64 in Moo Moo Farm as obstacles running loose starting on the second lap and later again on the third lap running loose from open gates. This happens again in Mario Kart DS as Moo Moo Farm reappears in that game.
Mario Kart: Double Dash!!
Mario Kart DS
Mario Kart Wii
Moo Moos reappear in Mario Kart Wii as an obstacle of a new course called Moo Moo Meadows. They run loose starting on the second lap and again on the third lap running loose from open gates again. They make a cameo on Moonview Highway in Mario Kart Wii on a sign saying "Moo Moo Dairy".
Mario Kart 7
Mario Kart 8 / Deluxe
In the Nintendo Switch game, Yoshi, they appeared in the E3 2017 trailer as cardboard.