There have been a variety of Muppet cows in various productions, including Sesame Street and The Muppet Show. One of the most prominent cows is Gladys the Cow. Other cows on Sesame Street included Natalie (a black and white cow), and a dark brown/red cow.

Occasionally, the Muppets mistake horses for cows. Such confusion occurs during the "Camelot" segment from The Muppet Show episode 305, "A Four-Legged Friend" (where Jim and Jerry ride cows but refer to them as horses), and the western sketch in The Muppets Go to the Movies.
Muppet cows