Cows are a continuously referenced Gaian species, exactly like the species on Earth.
Gaia Cows were a recurring creature with Gaia events, they first debuted in October 2006 for the Halloween event, subjected to being tipped and probed. When December 2006 arrived, there was an incident with a Zurg Ship, a tractor beam, a cow, some power lines and Santa Claus, resulting in a mind switch. Thus there was a cows mentality in the body of Santa, while Santa mentality was in the body of a cow.
When December 2008 arrived, Mrs. Claus was determined to have her husband back, using the power of x-mas cheer and a powerful machine called the Cheer Coil. So with the help of Gaians, Santa and the cow were returned to their proper places, but apparently the former Santa Cow was eaten afterward.