The equine sarcoid is a non-malignant and (usually) non-life threatening tumor on a horse's skin. They can vary vastly in appearance, and can be anywhere on a horse's body.
The next type is Verrucous Sarcoids. These appear anywhere on a horse, and look a lot like warts.
The third type is Occult Sarcoids. This type of Sarcoid is generally found on the neck or head of the horse, or sometimes in the inner thigh or groin area. They are flat and round, and may feel like a slightly thicker area of skin.
Another type is the Fibroplastic Sarcoid. These are common in the groin area, lower leg, or eyelid. They may weep, and appear like ulcers.
Malevolent Sarcoids are generally found in groups, and they are invasive. They are the most dangerous type, as they can involve the lymph system and are the most likely to lead to further health issues. They can be found anywhere on the horse's body.
Any of the aforementioned types of Sarcoid can be be found with one, or many of the other types. This is given the broad term "mixed Sarcoids".
Sarcoids are rather difficult to treat and nearly impossible to cure. If you are able to get rid of an outbreak, they are extremely likely to recur. Other than Malevolent Sarcoids, internal organs are most often not affected, and Sarcoids are not metastatic (they do not metastasize, or spread to other areas).
If you suspect your horse has Sarcoids, it's important to seek veterinary care immediately, and discuss the best course of action for your horse. It is also important to get a prompt diagnosis, to ensure it is not an invasive cancerous growth.