It is not known to evolve into or from any other Pokémon.
Along with Raikou and Suicune, it is one of the Legendary beasts said to be resurrected by Ho-Oh after the burning of the Brass Tower. Out of the three beasts, Entei is said to represent the flames that burned the Brass Tower.
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. Entei's forehead appears to resemble a kabuto.
An Entei is born whenever a new volcano appears. It is also said 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 said to represent the passion of magma. Entei can unleash massive bursts of fire capable of destroying anything it touches. Entei's fire is said to be hotter than the magma of a volcano.
When released from a Poké Ball, Entei unleashes a volcanic Fire Spin that deals a lot of damage to any opponents that get trapped in it.
In Melee, Entei is also featured in the second Trophy Tussle, where it is used as a stage akin to the Poké Floats stage. If the player is the last one standing, they receive an Entei trophy.
Melee trophy information
A volcanic Fire-type, Entei is hotter than liquid magma and as tough as nails. This powerful creature was one of the three Pokémon running around the Burned Tower of Ecruteak City. Entei is extremely difficult to capture, as it tends to wander all over the landscape, sprouting intense flames from its body.
Brawl trophy information
"A Volcano Pokémon. When Entei barks, a volcano erupts, and it's said that a new Entei is born every time a volcano appears. A proverb claims that the warmth of summer is created by Entei racing through the clouds. Entei uses Fire Spin, a flaming vortex that traps and damages foes for multiple turns."
3DS/Wii U trophy information
NA:One of the three Legendary Pokémon that rose from the ashes of the Burned Tower. It has a distinct white mane running down its back. When brought into battle, it unleashes a powerful Fire Spin attack that pulls in enemies, damages them, and then launches them as the fires peter out. Ouch!
PAL:One of the three Legendary Pokémon to rise from the ashes of the Burned Tower. It has a distinctive white mane running along its back. Its powerful Fire Spin attack will suck in any fighters nearby and trap them, dealing damage all the while. When the fire burns itself out, any fighters within are blasted off the stage.
Pokémon Ranger: Gordor considers Entei to be the best and most loyal of the Legendary trio. The player captures it when it is first unleashed in the Jungle Relic but it is still taken by the Go-Rock Squad, using their new Super Styler. Entei later acts as the final boss of the game.
When asked if Entei was based on a particular animal, Muneo Saitō described the creation process behind the Pokémon as "a hairy, rock-like body, volcanic smoke rising from behind its head, a king with a dignified beard... Entei's design was coming together... You could say it was basically like a lion...But I didn't want it to be simply an animal. So I settled on a silhouette that could look like a dog or a cat."
In many countries, the lion is regarded as a symbol of power and strength. Lions are also popularly known as symbols of fire. Its lion-like appearance and masses of fur make it appear similar the Balinese lion spirit Barong, a guardian spirit who kills the wicked witch Rangda in order to maintain the balance of good and evil.
Entei may be a combination of 炎天 enten (blazing heat) or 炎 en (flame, an alternate reading of honō) and 皇帝 kōtei (emperor). It can be written in kanji as 炎帝 (identical to the Chinese Yándì).