Polycephaly is a condition of having more than one

Supernumerary body part

Supernumerary body parts are most commonly a congenital disorder involving the growth of an additional part of the body and a deviation from the body plan. Body parts may be easily visible or hidden away, such as internal organs....

 head. The term is derived from the Greek stems

Word stem

In linguistics, a stem is a part of a word. The term is used with slightly different meanings.In one usage, a stem is a form to which affixes can be attached. Thus, in this usage, the English word friendships contains the stem friend, to which the derivational suffix -ship is attached to form a new...

 poly- (Anc.Gr. "πολύ") meaning 'much' and kephali- (Anc.Gr. "κεφάλι") meaning "head", and encompasses bicephaly and dicephaly (both referring to two-headedness). A variation is an animal born with two faces on a single head, a condition known as diprosopus

Diprosopus

Diprosopus , also known as craniofacial duplication , is an extremely rare congenital disorder whereby part or all of the face is duplicated on the head.- Development :Although classically considered conjoined twinning Diprosopus (Greek , "two-faced", from , , "two" and , [neuter], "face",...

. In medical terms these are all congenital

Congenital disorder

A congenital disorder, or congenital disease, is a condition existing at birth and often before birth, or that develops during the first month of life , regardless of causation...

 cephalic disorder

Cephalic disorder

Cephalic disorders are congenital conditions that stem from damage to, or abnormal development of, the budding nervous system...

s.

There are many occurrences of multi-headed animals, in real life as well as in mythology

Mythology

The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

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