There is a protein called myostatin that usually inhibits the growth of muscle. Through careful selective breeding of animals displaying a mutation that inhibits myostatin, farmers have been able to modify the genes of cows to produce some incredible double muscled animals. The most famous of these are the Belgian Blues.
Here’s a National Geographic Video about these amazing creatures.
It’s not only cows that display this amazing effect as very rarely it also affects dogs. Here’s Wendy the whippet who is around twice the weight of normal whippets and you’ll have to admit that she looks pretty ripped.