I reckon by the same route that people get to 15 foot tall bigfoot.
I should maybe state for the record that I do not, and never did, think that this a chupacabra. I'm almost certain it's not a coyote, but that doesn't make it a goat sucker. I think if a black-skinned golden retriever could mate with a hairless whitetail that the offspring would look a lot like this critter. Other than that, I'm clueless.
Pretty cool, though.