Entei is a massive lion-like Pokémon with some mastiff qualities. It is covered in brown fur with a long, thick mane covering its neck. It has grey plates surrounding the cape that makes it look as if smoke is coming out of a crater on its back, and a plate beneath the cream belly fur on its chest. Entei has white paws with apparently retractable claws and brown paw pads, and black cuff-like bands on its legs. It has a red face with a white plate that resembles a handlebar moustache on its face, and a three-pointed yellow crest on its forehead. Entei's head appears to resemble a kabuto, a samurai helmet.

It is said that an Entei is born whenever a new volcano appears. It is also said that whenever Entei roars, a volcano erupts somewhere in the world.