The Baby Cows appeared in the episode "Fun With Veal"
The rescue
When the class goes on a field trip to Farmer Denkins' ranch, they discover that a group of baby cows will be slaughtered and turned into veal. While Cartman begs for a sample of the baby cows, Stan Marsh, Kyle Broflovski and Butters Stotch are all horrified and plot to wanted to save them. Breaking into the farm, they steal the cows and hide them up in Stan's bedroom. The boys stay holed-up in the room, for several days, defying their parents and being forced to suffer ridicule and assocation with hippes.
They would have starved, had Liane Cartman not snuck them food. However, while the other boys ate from the assorted meat products supplied by Ms. Cartman, Stan refused, eating only apples and fruit. Oddly enough however, Stan began to develope odd sores, that were later discovered to be viginas.
The boys negotiated with the police, all of which led up to them trying to escape to the airport in a truck driven by Michael Dorn, dressed up in Star Trek Klingon make-up. The boys are double-crossed however, and the cows are returned to Farmer Denkins. However, due to the fact they were allowed to mature as far as they had, and the fact that veal was changed to "slaughtered baby cows", they could not longer be turned to veal and thus allowed to grow up and live out their lives as normal cows.