I watched a television program once that said that this genetic mutation in a goat was probably the factor which developed into the unicorn myth. According to the program that from a distance a goat could look like a small horse or a baby horse. It sounds viable to me.

It does sound plausible. Especially since I read somewhere recently that all of our different breeds of domestic animals came from mutations originally. For instance all the different breeds of cats.

That's very true. Most of the different breeds of domestic animals are our own controlled mutations of the originals. Cats, Dogs, Horses, most have been bred that way. We call it "genetics" now, but a hundred years ago, it was simple animal husbandry. Breeding them for specific purposes and needs to suit neccessity.