Deino is a small blue skinned quadrupedal Pokémon with black fur covering its head and neck which conceal everything but its mouth from view. The fur around its neck also has six purple spots encircling it and the hair on the top of its head takes the shape of a single small horn. It also has a short tail.
Deino have the vigor and energy to bite constantly and without stopping.
Because they are blind, Deino will bite at anything. They never stop biting, and are therefore dangerous to approach without taking precautions. They will also ram anything, ensuring that they always have bruises all over them.
Deino primarily lives in caves with abundant water sources.
According to interviews with Sugimori in Nintendo Dream, Deino and its evolutions were originally intended to be cyborg dragons - part tank and part dragon. They were then scrapped until late in the game when they were revived and re-made to be based on Yamata no Orochi.
Deino is based on a dragon, specifically an infant form of Yamata no Orochi. Its blindness and the fact that it is only found in the Victory Road cave may mean it suffers from subterranean blindness. Deino may also be partially based on various cave salamanders due to their blindness and that they are commonly found near bodies of water in caves.
Deino may come from dino, a slang term for a dinosaur, and ein, German for one. Deino could also be derived from the ancient Greek word deinos, which means terrifying. Monozu probably derives from mono, a Greek prefix meaning one, and 頭 zu, Japanese for head, its full name literally reading "one head".