These animals have natural mutation of the gene that codes for myostatin. Pics of those mega cows were used to sell myostatin blocking supplements, which were all the rage for about 10 minutes as some of you may recall.
They are the results of good old selective breeding which we have done for thousands of years on one animal or another.
It's funny that people will often refuse to see how genetics rules their physique ultimately, but that's another topic for another day.
http://www.youtube.com/watch?v=Nmkj5gq1cQU
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