The condition of having more than one head is called polycephaly. Tricephaly is the condition of having three heads. Practically all confirmed cases of polycephaly consist of animals with two heads, called bicephaly. The Tech Museum of Innovation explains that most polycephalic animals are conjoined twins.
Two-headed snakes are relatively common. Most only survive for a few months in the wild, but a few have been known to survive for years in captivity. Some examples of multiple-headed snakes appear to be hoaxes. For example, Hoax Slayer has posted pictures of a three-headed and a seven-headed cobra that are clearly modified images of a perfectly normal one-headed cobra.