Adjective. Early 19th century.
[From BI- Latin (earlier dui-, cognate with Greek DI-, Sanskrit dvi-) = twice, doubly, having two + Greek kephale head + -OUS.]
Bicephalism, while common in mythology, religion and heraldry, is rare in nature and always the result of a congenital disorder. While bicephalous animals have a poor survival rate in comparison to monocephalous ones, there are examples that survive into adulthood and many museums around the word carry exhibits of preserved two-headed animals. Interestingly, the bicephalous eagle is a ubiquitous and truly ancient symbol with examples in Sumerian, Hittite and Babylonian cultures.
Related (but distinct) words are polycephaly (having more than one head) and diprosopus (having more than one face).