Polycephaly is the condition of having more than one head. The term is derived from the Greek stems poly (Greek: "πολύ") meaning "many" and kephalē (Greek: "κεφαλή") meaning "head". A polycephalic organism may be thought of as one being with a supernumerary body part, or as two or more beings with a shared body. Two-headed animals (called bicephalic or dicephalic) and three-headed (tricephalic) animals are the only type of multi-headed creatures seen in the real world, and form by the same process as conjoined twins from monozygotic twin embryos.In humans, there are two forms of twinning that can lead to two heads being supported by a single torso. In dicephalus parapagus dipus, the two heads are side by side. In craniopagus parasiticus, the two heads are joined directly to each other, but only one head has a functional torso. Survival to adulthood is rare, but does occur in some forms of dicephalus parapagus dipus. There are many occurrences of multi-headed animals in  mythology. In heraldry and vexillology, the double-headed eagle is a common symbol, though no such animal is known to have ever existed. Occurrences
Two-headed people and animals, though rare, have long been known to exist and documented. Occurrence in humans
In humans, as in other animals, partial twinning can result in formation of two heads supported by a single torso. Two ways this can happen are dicephalus parapagus, where there are two heads side by side, and craniopagus parasiticus, where the heads are joined directly.
There is some speculation that the inbreeding of snakes in captivity increases the chances of a two-headed birth. One or two beings? It is difficult to draw the line between what is considered "one animal with two heads" or "two animals that share a body". Abigail and Brittany Hensel, born in 1990, were given two distinct names at birth. They identify as two people, and are recognised as two people legally and socially. On the other hand, Syafitri, born 2006 in Indonesia, were given one name by their parents because they only had one heart. In early Germany, conjoined twins that could not be separated were legally considered one person. Millie and Christine McKoy were often referred to in the singular, including by themselves, with the name "Millie-Christine", as well as plural.In Peter Mogila’s 17-century Catechism, the following instructions are given for baptism of polycephalic infants: should there be distinct heads and distinct chests, this means there are separate people each of whom must be baptised normally; if the heads and chests are not completely distinct from each other, however, one person must be baptised normally but baptism of the other(s) should be preluded by the formula "if not already baptised".With other animals, polycephaly is usually described as "one animal with two heads". One of the heads, especially in three-headed animals, may be poorly developed and malformed, and not "participate" much.