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. As it is blind, it will bite at and eat anything that moves, and it has the vigor and energy to bite constantly and without stopping. It never stops biting, making it dangerous to approach without taking precautions. It will also ram anything, ensuring that it always has bruises all over it. As seen in The Lonely Deino, it may rear up on its hind legs when happy. Deino primarily lives in caves with abundant water sources.
A Team Plasma Grunt used a Deino in a flashback in Unforgettable Memories. It fought against Hugh's Purrloin and Trapinch after Hugh refused to listen to them when they convinced him that he was causing pain to his Pokémon. Deino was able to weaken Purrloin, forcing Hugh's sister recall it to its Poké Ball, which was then taken by two Grunts afterwards.
Of all the Pokémon that evolve by level alone, Deino evolves to its final form later than any other Pokémon, evolving at level 64.
Deino also evolves into its second form at level 50, later than any other Pokémon that evolves by leveling up and can evolve again.
According to interviews with Ken Sugimori in Nintendo Dream, Deino and its evolutionary family were originally intended to be cyborg dragons with aspects of tanks. These Pokémon designs 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 may be a combination of dino (short for dinosaur) and ein (German for a, and part of the German for one, eins). It may also derive from δεινός deinos (Ancient Greek for terrible). It may also refer to dinosaurs and other prehistoric animals such as Deinosuchus and Deinonychus. Deino is also the name of one of the Graeae, the three sisters in Greek mythology who shared one interchangeable eye, leaving two blind while one could see.
Monozu may be a combination of mono (prefix meaning one) and 頭 zu (head). Its full name literally means "one head".