Deino is a small blue quadrupedal Pokémon with black fur covering its head and neck, concealing everything but its mouth from view. The fur around its neck also has six purple spots encircling it, which resemble bruises. The hair on the top of its head takes the shape of a single small horn. It also has a short tail that appears to be bitten off. Deino's body is covered in wounds.
As Deino is blind, it figures out its surroundings by ramming and biting. It primarily lives in caves with abundant water sources but little food source, so it will eat anything that moves, and memorizes the scents of its favorite things that it has tasted. It has the vigor and energy to bite constantly, making it dangerous to approach without taking precautions. As seen in the anime, it may rear up on its hind legs when happy.
Of all the Pokémon that evolve by level alone, Deino evolves to its final form starting at the highest level, evolving at level 64.
According to interviews with Ken Sugimori in Nintendo Dream, Deino and its evolutionary family were originally intended to have aspects of tanks. These Pokémon were scrapped until late in development, when they were revived and remade to be based on Yamata no Orochi.
Deino is the only Pokémon whose English name's letters are in alphabetical order.
Deino and Zweilous are the only Pokémon who have Hustle as their only Ability.