I was always curious how long it takes that type of mutation to become a distinct strain (e.g. how many generations from the first sheep herder that saw a sheep with a fat tail and decided, either consciously or otherwise, to breed it and continue to select offspring with fat tails until it no longer required selective propagation to persist).

If we're talking sheep, then I have to put out a shout for my local breed - the Romney Marsh sheep (the sheep that conquered the world!) - https://en.wikipedia.org/wiki/Romney_sheep