From Muppet Wiki

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.

The Muppet Show

• Episode 121: Twiggy
• Episode 211: Dom DeLuise
• Episode 224: Cloris Leachman
• Episode 322: Roy Rogers & Dale Evans - several cows
• Episode 408: Arlo Guthrie - there were six different cows and a bull.
• Episode 418: Christopher Reeve
• Episode 512: Melissa Manchester - a cow falls on The Newsman

Sesame Street

• Gladys the Cow
• Natalie, the Cow who jumped over the moon in Sesame Street News Flash
• Back-up singers for How Now Brown and the Moo Wave

Elsewhere

• National Wildlife Federation - Bessie