Two-headedness happens from the failed separation of monozygotic twins (the same misstep in the process that makes conjoined twins). Each head typically has its own brain that can both control its shared limbs.
This cat is treated like two cats by its owner. Named Frank and Louie, it kind of personifies a two-creature, one side looks Puss in Boots cute while the other seems sneaky and sinister. Image: AP
Pigs can have two heads too! Though it doesn't mean more belly, it does have an aw, shucks cuteness to it, doesn't it?
Two-headed dogs can happen naturally but mad Soviet scientist Vladimir Demikhov actually performed dog-head transplants on man's best friend through surgery.
Cyclops cow is very scary looking. Image: AP
Found by April Britt in Massachusetts, this bird is a baby cardinal with two heads and three beaks. It's unlikely that the two-headed bird would live long in the wild because animals with abnormalities usually fare poorly in the ruthless world. Image: April Britt via the Daily Hampshire Gazette
This two-headed scaly beast has heads on opposite sides of each other. Found in 2001 in Thailand on a crocodile farm, the crocs are more Siamese twins than polycephalic. Either way, they still have two heads and though two-headed reptiles are more common than mammals (polycephalic reptiles don't even look that weird to me), these crocs are just insane.
This two-headed brown trout is either a result of nature's cruel randomness or a mutation caused by a big corporation who could give a crap about the environment.