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Polycephaly is a condition of having more than one head. The term is derived from the stems poly- meaning 'many' and kephal- 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. In medical terms these are all congenital cephalic disorders.
There are many occurrences of multi-headed animals, in real life as well as in mythology. In heraldry and vexillology, the double-headed eagle is a common symbol, though no such animal is known to have ever existed.
Bicephalic or tricephalic animals are the only type of multi-headed creatures seen in the real world and form by the same process as conjoined twins: they all result from the secondary union of two originally separate monozygotic embryonic disks. One extreme example of this is the condition of craniopagus parasiticus, whereby a fully developed body has a parasitic twin head joined at the skull.