Although the skin is the most visible of the horse's body structures it is also the most easily overlooked! The skin is very important - it provides a strong barrier to challenges from outside the body and plays and important role as part of the immune system, helps control body temperature, and makes vitamin D. In certain parts of the body the structure of the skin is changed to perform specific functions. For example the hoof, chestnut and ergot are modified skin structures, and the skin of the eyelids is much thinner than on the back.
In spite of the exposed position of the skin, it is usually remarkably free of disease. However the skin is also the organ most likely to be damaged accidentally - cuts, bruises and burns are common! Skin diseases are common in horses, but many have very similar appearances. Skin lumps are extremely common in horses and can be broadly divided into cancerous and non-cancerous conditions. The most common tumours are sarcoids and melanomas. Other lumps could be allergic reactions to insect bites, plants or less commonly medicines or internal disease.
Horses also suffer from common parasitic skin conditions such as lice and mites, as well as other bacterial and fungal conditions like rain scald, mud fever and ringworm to name a few; these conditions are all treatable so you should call your vet as soon as you suspect your horse is suffering from one of them. Other conditions you might encounter include loss of pigmentation at a site of injury or inflammation, or vitiligo, where there is a progressive loss of pigment in the skin; there is no treatment for these, but remember that any depigmented skin should be protected from exposure to sunlight because pale skin is more susceptible to sunburn and the risk of skin cancer at these sites is higher.
Early veterinary attention gives the best chance of a good, uncomplicated diagnosis and appropriate treatment. Be warned though that many skin disorders are complicated by the use of the wrong treatment or the incorrect use of the right treatment.