Cow

Cows are a race of mammals that roam Hyrule and its surrounding countries. They walk on all fours and are generally docile, often kept at farms such as Lon Lon Ranch. Although their most frequent (and perhaps only) vocalization is a sound which sounds like "Mooo", Link was able to understand their language when in Wolf form or when wearing the Mask of Truth.

Cows are the source of Milk, which has great healing properties. When Link would play Epona's Song for the cows, they were always happy to give him a refill for his milk bottle.

Ocarina of Time Master Quest
In the Master Quest of The Legend of Zelda: Ocarina of Time, cows made much more frequent appearances than in other Zelda games. Specifically, in Lord Jabu-Jabu's belly, cows were seen protruding from the walls and ceiling, without explanation.