I am posting this picture because I am intrigued about it, and haven't yet been able to find out whether the practice was common or not. The picture below was sent in to Riding Magazine in the 1930s by Rupert Isaac of Ipswich, and shows a stallion's tail attached to a cottage in the village of Kersey.
Isaac wrote: "....its meaning to the villagers is plainer than any signboard; for when your horse goes lame or your cow has colic, you seek the stallion's tail -- the device of the veterinary surgeon. When the first tail was hung out is a matter for conjecture, but the "horse doctor" was one of the earliest craftsmen, and Kersey is old indeed..."
There is a photograph from 1956 in the National Archives which shows, it says, the farrier's trademark of a horse's tail on a house in Kersey. Don Lemon was the local horse doctor in Kersey during the 1940s - he had no veterinary qualifications, but his house was marked by a horse's tail, according to an article in The Independent, 23 January 1993 about the sale of some houses in Kersey. A more recent advertisement for a holiday cottage in the area mentions a fox's tail: whether the original is now so matted that's what it looks like, or whether the tail has been changed and no one now remembers what it ought to have been I do not yet know.