Sarcoids are similar to that of a wart. There are 6 different types, the most common being 'mixed' and 'occult'. They can be passed by flies, and they are a viral problem. I had the vet out to one of my Tbs last weeks who has an occult sarcoid, he has ordered cream (Cytotoxic ointment very poisonous and kills sarcoid cells) from Bristol Veterinary Laboratories as it has to be specially made up in our country. The cream he is having has to be applied by the vet as it is chorosive and very abrassive. If you have an occult or mixed sarcoid, if left alone and not caught by tack or other forms of contact, they don't usually spread and will just look unsightly but I would get your vet to investigate and apply the medication that is used in the USA to get rid of them.
Here are the various types;
Occult=common on hairless parts of the body, appear as round swellings, with grey scaly surface, if undisturbed remain unchanged for many years.
Verrucose= have a warty appearance and often develop into thicken skin growths, they are slow growing but if disturbed can become aggressive and localy invasive.
Fibroblastic= involve both dermal and subdermal layers of skin, they extend much further than appears. They are fleshy and have an aggressive appearance, rapidly ulcerateand often associated with surgical or trauma wounds.
Malevolant= are very aggressive and spread along the lymphatic chanels causing large ulceratied masses commonly found on the face and internal thigh.
Nodular= they appear to be under the skin, well rounded and have a well defining border. The overlying skin is thin and shiny.
Mixed= the most common of sarcoids, they have the appearance of the occult type, and verrucose and nodular types too.
They are thought to be a virus, and thought to be the result either of an earlier non-productive infection with the virus that causes juvenile warts, or infection with that which causes warts in cattle. Can be singular or multiple on the body and don't often go away on their own.
Hope this has helped, do call out your vet. :)
A horse belongs in a stable not on a table!