What is Additive Monster?
ADDITIVE MONSTER: In contrast with the composite monster, mythologists and folklorists use the label additive monster to describe a creature from mythology or legend that has an altered number of body parts rather than body parts from multiple animals added together.
For instance, the Scandinavian Ettin, a troll or giant with two heads, is an additive monster. Sleipnir, the magical horse in Norse mythology, is a regular horse, except it has eight legs. Deities and demons in the Hindu pantheon often have multiple arms or eyes. The term has also been loosely applied to fantastic creatures that have modified limbs as well.
For instance, the gyascutis is a fantastic medieval beast that resembles a sheep, except its limbs vary in length. Its front legs are drastically shortened, and its hind legs are drastically lengthened, which allows it to remain level as it grazes on the incline of steep hills.