Two-headed creatures aren't just something out of a sci-fi story. Double-headed animals do exist in the natural world. Having two (or more) heads attached to one body is a condition referred to as polycephaly. That intimidating word has pretty simple origins: the Greek poly- meaning "much," and kephali-, which means "head."
Though it's an extremely rare occurrence, scientists who study and catalog animals have come across quite a few instances of polycephaly. We even use the two-headed animal as a symbol. For example, a two-headed eagle often stands for something grand and historical. It was also the mark of the Byzantime Empire.
This list of two-headed animals aims to separate the myth from the unbelievably true.