Drampa is based off of Chinese dragons, which can fly but have no wings. Bulbapedia says it is most likely inspired by Zhulon, a beast with the body of a dragon and the head of an old man. Drampa is, of course, the ultimate old man Pokémon.
Dunsparce is harder to make an argument for, but it's Bulbapedia page explains it pretty well: Its (sort of ) serpentine physique and small wings suggest that it may take after the Feathered Serpent — perhaps more commonly known as the Aztec deity Quetzalcoatl — in Mesoamerican religions. It makes sense that a snake with wings could roost, yes?
Of course, there's also the classic answer of "Game Freak logic." What they says goes, and we mortals can only swim with the current.
I hope this helped!