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)
Entei was a main character in Pokémon 3: The Movie. He is an illusionary Entei that appeared to Molly Hale after she made a wish to the Unown, serving as a replacement father for her while her real father was trapped in the Unown dimension. He did everything Molly asked for, even kidnapping Delia Ketchum so she can be Molly's mother and providing her with Crystal Pokémon for her to battle Brock and Misty with when they entered the tower that Entei had created to rescue Delia. When Ash attempted to confront Entei to get his mother back, his Totodile and Cyndaquil were rapidly defeated, with Pikachu only lasting longer without fairing any better, and even Charizard was only able to hold Entei off without actually defeating it. However, when Entei was about to kill Charizard, Molly finally decided that she wanted things to be real again. Accepting Molly's decision, Entei helped Pikachu and Charizard defeat the now out-of-control Unown, subsequently bidding Molly farewell and assuring her that he would always be in her dreams.
