Multiple realities(covers information from several alternate timelines)
A cow is an Earth mammal used in agriculture, primarily for its milk. Cows are also rasied to be slaughtered and eaten.
In an alternate reality version of 2255, a figurine of a cow could be seen in the Shipyard Bar. (Star Trek)
The Bringloidi colonists brought a cow aboard the USS Enterprise-D in 2365. (TNG: "Up The Long Ladder")
In 2376, Tom Paris turned Harry Kim's holodeck date, Maggie O'Halloran, into a cow as a prank, unknowingly causing the people of the Fair Haven holoprogram to believe that Paris and Kim were evil spirits. (VOY: "Spirit Folk")
Cows were used in hamburger meat. (ENT: "Carpenter Street"; VOY: "Fury")