Friesian, Gypsy Vanner, PRE (Purebred Spanish horse, formerly called Andalusian), Lusitano, American Saddlebred & Arabian are probably the best known breeds capable of growing extravagantly long manes and tails. Many of these horses are carefully kept so that the manes and tails do grow as long as possible. read more
Any horse breed except old-fashioned "typey" Appaloosas will grow long manes and tails if properly groomed and cared for. The reason it looks as if Thoroughbreds, warmbloods, many draft breeds and such don't is that for various reasons of function and/or style the manes are kept pulled short. read more
Horses with short manes usually have their manes combed, while horses with longer manes are usually groomed with a human hair brush or a stiff dandy brush. Horses with extraordinarily long manes may have their manes hand picked to remove tangles. For a horse show, the horse is generally bathed, and this includes the mane. read more