The causes of skin growths in dogs range from mild to serious conditions. Smooth, round, hairless, fluid-filled nodules are sweat gland cysts. These are common and often removed surgically.
Hard nodules on the back or groin that drain and crust over are often a reaction to the use of too many corticosteroids. They are typically treated by slowly weaning the dog off the medication. Firm, fluid-filled nodules on the ears are sometimes the result of leaking blood vessels, particularly when dogs have infections or long ears.
In certain cases, skin growths form around parasites such as the larvae of the Cuterebra fly or the Guinea worm. Also, as in humans, some skin growths are the result of cancer.