The pseudo-legendary dragon of Unova, Deino is a Dark/Dragon type, and the only pseudo-legendary Pokemon to have Dragon as a secondary type. The names of the Deino line derive from the German words for the number of heads each one has. Deino contains Ein, German for 'a' and part of the German for 'one,' Eins. Zweilous contains Zwei, German for 'two.' Hydreigon contains Drei, German for 'three.'





Being blind, Deino tackles and bites to learn about its surroundings. This is similar to sharks, who bite things not always out of hunger, but also to find out what they are. Deino will eat anything that moves, so its trainer has to be very careful. Deino's body is covered in wounds sustained from its rather reckless method of approaching the world.





When Deino evolves into Zweilous, it continues its biting passion by eating far too much than it ought to with its two heads. Whichever head eats the most gets to be the leader, so Zweilous's heads compete with each other for food and don't get along with each other. Zweilous are nomadic, moving on to greener pastures once they've eaten everything they can find in their previous territory.





While Zweilous has two functioning heads, Hydreigon has only one, as the two heads on its arms do not have brains. Game Freak has never determined what happens to the second head, but it may be presumed that the head that is in charge when Zweilous evolves is the head that survives. Hydreigon, like Deino, will attack anything that moves, and its three mouths will devour anything. It sees anything that moves as its opponent, which means that its trainer might not want to take it on walks around the neighborhood until it's learned to keep its jaws to itself outside of battles. But as Iris proved in the episode A Village Homecoming!, taming Hydreigon can be done.





Notably, a very different Hydreigon appeared in Pokemon Mystery Dungeon: Gates to Infinity. While initially presented as a foe by Munna to the player, Hydreigon actually proves to be the incarnation of the Voice of Life, the manifestation of nature's collective will to exist. Why the Voice of Life takes the form of Hydreigon of all Pokemon is unknown, but it may be a metaphor of many entities (respresented by multiple heads) uniting with one mind. While the Voice of Life is polite and well-mannered, it does retain the voracious appetite all Hydreigon share, but he sates it (fairly) civilly at the Swanna House inn's restaurant.





According to interviews with Ken Sugimori in Nintendo Dream, Deino's original concept was of a cyborg dragon with elements of tanks. While the original Deino line was scrapped, they were revived later in development and remade with the designs they have today. Deino's evolutions, Zweilous and Hydreigon, have tank tread markings on their bodies, remnants of their original forms.





The final concept for Deino's evolutionary line was based on the Yamata no Orochi. The Yamata no Orochi was a legendary eight-headed and eight-tailed Japanese dragon that ate human flesh and was eventually killed by the Shinto storm god Susanoo. Susanoo destroyed the dragon by preparing eight large vats of liquor for the dragon's eight heads, getting the entire creature drunk. When the Yamata no Orochi finished drinking, it lay down to sleep, and Susanoo chopped it into pieces, turning its own overindulgence against it. Interestingly, there was a sword inside the Yamata no Orochi's tail, which may explain why the Deino line have tails that appear to be broken at the tip.





The Yamata no Orochi is not the only multi-headed dragon in mythology. The Hydra of Greek mythology is the most famous, but the Zmey Gorynych of Slavic mythology is the closest to Hydreigon, having exactly three heads. It battled the warrior Dobrynya Nikitich for three days and three hours, and was eventually slain. Zmey Gorynych was a parent, and its dragon pups, like Deino, were biters, with one pup paralyzing Dobrynya's horse by biting it on the leg.





Deino evolves into its second form at level 50, later than any Pokemon that evolves by leveling up and can evolve again. Also, Deino evolves into its final form later than any other Pokemon, evolving at level 64. Given how brutal Hydreigon can be, perhaps it's for the best.





Competitively, Hydreigon was in Smogon's OverUsed tier upon its introduction in Gen 5, but dropped to UnderUsed with the introduction of the Fairy-type, which Hydreigon is doubly weak against and unable to even hit with Dragon moves. Even before the Fairy type, Hydreigon's main weakness was its low Speed stat, but its large array of damaging moves and lack of counters in the OU tier made it a surprisingly viable pick. That changed in Gen 6, when Fairy-types dropped Hydreigon down to UnderUsed, where it remains to this day.







