As of Generation VI, Drapion and its pre-evolution's Egg Group (Bug and Water 3) are unique.
Skorupi and Drapion's Origin:
>Drapion is based on a scorpion, and has some characteristics that resemble a vinegarroon (a similar arachnid). Its Water 3 Egg Group designation may be a reference to the ancient group of arthropods known as sea scorpions that used to inhabit the oceans and may have been the ancestors of modern scorpions.
Source: Skorupi and Drapion.