Physical:
* Large
* Bovine
* Mammals
* Herbivores
* Ruminant
* Hooved
* Spotted (for some breeds)
* Black and white (for some breeds)
* Brown (for some breeds)
* Muscular
* Heavy
* Sturdy
* Gentle
Behavior:
* Peaceful
* Docile
* Grazing
* Mellow
* Calm
* Chewing
* Mooing
* Udderly (informal, playful)
Farm/Agricultural:
* Dairy
* Beef
* Livestock
* Pasture
* Milking
* Farming
Other:
* Sacred (in some cultures)
* Friendly
* Curious
* Silly
Figurative:
* Moo-licious
* Udderly delightful
You can also use words to describe the breed of cow, like Holstein, Jersey, Angus, etc.
The best words to describe a cow depend on the context and what you want to emphasize.