It's a random mutation that shows up in the wild. But it happens on somatic tissue and not germic meaning that it is not heritable In any gas gun that we have seen. Heck some calico show up after a year of being a normal snake. Then one shed one day it loses scale pattern. There are very very few captive born and bred calico. I know of 2. The rest have all been wild caught which is more common and hence why most are super aggressive