The short-legged Ancon sheep is descended from a mutant ram discovered in Dover, Massachusetts in 1791. Why did this mutation become established?

It was established because Ancon sheep mutation was noted in the excavated remains in the UK which dated as early as 1500 AD and later found in other parts of US in 1700-1900. It only indicates that the mutation exists even before the Dover ram discovery in 1791. Dwarfism mutation with a pronounced short leg in Ancon sheep was caused by constant crippling.