I've never heard of them on horses just people. I know its gross but I get them occasionally and used to have the Dr. Remove them. Now I just use a razor and sort of shave them off myself
They are similar to a wart and aren't really a health risk.
I'm sure they are the same in horses.
I just found this on http://pet.justanswer.com
"Warts and Sarcoids have confused lay horse people and Vets alike for many years. Even the professional horse people can be unsure. The Papilloma Virus that causes warts generally seen on the face and ears sof horses, tends to be in young horses. However, they are not limited to the face or ears and can some times appear on the body itself. Normally, they will resolve in 6-9 months. If the horses immune system has been unable to beat this, they this will continue to be occurring. Upping the quality of feeed, adding coat condtioning suppliments and keep the brushes and halters etc cleaned with lysol will help. These are contagious. A professional Vet familiar with freezing or laser can do this for you. High Performance Horses in the US on the Grand Prix do it all the time. However, have you Vet check these to be sure that they are not Sarcoids. I don't think they are, because these tend to break open and ulcerate. That is a completely different problem."
This is from the Merck Veterinary Manual:
"In horses, small, scattered papillomas develop on the nose, lips, eyelids, distal legs, penis, vulva, mammary glands, and inner surfaces of the pinnae, often secondary to mild abrasions. They can be a herd problem, especially when young horses are kept together, but regress in a few months, as a foal’s immune system matures.
When they develop in older horses, they often persist for >1 yr. So-called aural plaques are also thought to be a flat form of papilloma (verruca planum). Equine papillomas are disfiguring but benign. They need to be distinguished from verrucous equine sarcoid ( Soft-tissue Sarcomas