Its a common thing with whippets grehounds and lurcher types. Some of my whippets have had a bald bit on their tails although I have found that with mine it I have never seen it on the ones with dark coloured tails. I never seen is on Owen Tegan or Angel or the fawn twins but Bandit and Jasper have had it. I just left it alone and it eventually went. I also asked other people with whippets and they said theirs had had it. You often see them at shows with a bald patch on their tails.