It happens sometimes in nature that the splitting of a twin embryo goes wrong. In the case of animals, the effect can be quite stunning and rather endearing.
The 1972 movie "The Thing With Two Heads" is probably one of the weirdest premises and insane casting I've ever seen as Ray Milland is a wealthy dying man who has his head transplanted onto another man's body, played by Rosey Grier.
Sesame Street created a 2-headed monster for the children's delight.
It is always a wonder when nature can give us double the adorableness, like in the case of Abby and Brittany. These two are a wonder and a joy.