This is a fistulated cow, and that hole in its side leads to its stomach. This window has a number of uses beyond turning your cow into a submarine, from research to cow medicine. For example, when cows develop bacterial deficiencies they can be replenished by just scooping some out of the local window-stomach. 

Cows aren’t the only ones who can get fistulated; listen as RadioLab describes how a young man, shot in the stomach, ended up with a permanent hole to his digestive system. And how that led to some of the earliest knowledge on how our guts work.