It is not known to evolve into or from any other Pokémon.
Along with Raikou and Suicune, it is one of the Legendary beasts 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.
This section temporarily contains information copied directly from Bulbapedia, and needs to be rewritten so not to be classified as plagiarism.
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 covers its muzzle and resembles a handlebar mustache. 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.