Entei did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Entei is a Legendary Fire-type Pokémon from Johto. It is one of the legendary beasts, along with Raikou and Suicune. Out of the three beasts, Entei is said to represent the flames that burned the Brass Tower.
Entei is a massive lion-like Pokémon with some mastiff qualities. It is covered in brown fur with a long, thick mane covering its neck. It has gray plates surrounding the cape that makes it look as if smoke is coming out of a crater on its back, and a plate beneath the cream belly fur on its chest. Entei has white paws with apparently retractable claws and brown paw pads, and black cuff-like bands on its legs. It has a red face with a white plate that resembles a handlebar mustache on its face, and a three-pointed yellow crest on its forehead. Entei's head appears to resemble a kabuto, a samurai helmet.
It is said that an Entei is born whenever a new volcano appears. It is also said that whenever Entei roars, a volcano erupts somewhere in the world.