Entei is a massive, leonine, quadrupedal Pokémon with some mastiff qualities. It is covered in brown fur and a long, light gray cloud of smoke running along its entire back. It has gray plates on either side of the cloud and a plate beneath the cream belly fur on its chest. Entei has gray paws with brown pads and black cuff-like bands on its legs. Both front paws house three small, white retractable claws, which can be seen in battle or in Pokémon Camp. Its main faceplate is red and shaped like fins or a six-sided star. A gray plate, resembling a horseshoe mustache, covers its muzzle. It also has a yellow crest with three sloped points on its forehead and red eyes.

It is said that an Entei is born whenever a new volcano appears, and that whenever Entei roars, a volcano erupts somewhere in the world. Entei races across grasslands at high speeds. It does this due to being unable to contain its extreme power. Entei is believed to represent the passion of magma, and is claimed to be the bringer of the summer's warmth. Entei can unleash massive bursts of fire capable of destroying anything it touches. Its fire is said to be hotter than the magma of a volcano. Entei loves Entei Treats made by Snacksworth, who once ran into and chased it in the past. Gouging Fire shares a resemblance to Entei. It is theorized to be an ancient Entei regenerated from a fossil.

As mentioned in the Sleep Style Dex, Entei is known to sometimes leak flames from its mouth. It also leaks flames while it sleeps as well. After taking a good rest, Entei will sweep across the land, expelling waves of fire similar to an erupting volcano. Entei is known to enjoy eating Entei Biscuits. Entei can be lured over to a sleeping spot using the Entei Incense. Entei has three different sleep styles it sleeps in, they are called Energy-Storing Sleep, Meditative Sleep, and Volcano Sleep.

(Source: Bulbapedia)