You may be seeing the birth sack not the cows rectum.
a gunny sack is a sack made of gunny or burlap
The sack worn by mountain men around their necks was called "a bullet pouch."
Twelve cows can be called a flink, a dozen head or a herd of cows.
amniotic sack
Females are called cows and the males are bulls.
The pink sack is called the placenta.
possibles sack
Sac
There are a whole pile of animals that have females called cows:ElephantsBuffaloBisonElkMooseCaribouDomesticated BovineWhalesRhinocerosesWalruses
Isn't it a ballsack? No, it's a bindlestiff (bindle for the sack, stiff for the stick that holds it.)
A sack