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 quadruped with some mastiff qualities. It is covered in brown fur and a long, light gray cloud of smoke for a mane running along its entire back. It has gray plates on either side of the mane 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. 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. As a whole, Entei's head 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.
In this manga, Entei's fire has a healing effect, burning away disease and evil auras. It is also the move capable of permanently melting away the Masked Man's moving ice sculptures, which typically repair themselves from any attack.
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.
Entei may be based on Chinese guardian lions, or Lions of Foo, creatures that resemble fully grown male lions. They are often confused with canines and incorrectly called "Foo Dogs" by Westerners. Their original significance was serving as guardians (like gargoyles upon churches) at Buddhist temples. The image of a ferocious beast would scare intruders away. This correlates with Entei being seen as a guardian in the third movie, protecting Molly and her allies.
Like a real lion, Entei boasts a regal mane, long retractable claws, and a strong, bulky, yet agile body. Its face is short and bears resemblance to a lion's skull — short and wedged, but with the unique flatness that ShiShi statues also bore. The yellow design on its head could be interpreted as a stylized crown or a Kabuto helmet. In China, the lion is regarded as a symbol of power and strength. Lions are also popularly known as symbols of fire. However, Entei also resembles large short-muzzled dogs, such as a mastiff. Entei's brown shaggy fur and white paws and muzzle are similar to a St. Bernard, a large dog known for its strength. Its lion-like appearance, mask-like face, and masses of fur make it look like 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's roar is known throughout the Pokémon world as a sign of a new volcano erupting, and a new Entei being born. Entei may also have some inspiration from the Aterui, the chief of the Emishi, who was later labeled as a demon called 'Aka-gashira' (Red-Head). It also has the appearance of a volcano, its head is volcano-shaped and has a bright yellow eruption-flame on the top, and the red magma is seen on the side of its face, it has volcanic smoke leaving from the back and has two spiky mountains on the back.
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).