Entei

was a horse yōkai that obeyed only the most powerful of yōkai.

History
The anime gives some insight into Entei's background which was completely unexplored in the manga. Entei first appears to a rather powerful halberd-wielding ogre demon named Rengokuki who fought off hordes of demons with ease and had a strong will. Recognizing his strength, Entei offered him his servitude and the two become a feared duo. Some time later, the demon was pacified by a powerful holy seal by monk Shinsen. The seal is then destroyed by Hakudōshi years later.

During the story
Entei reunited with his old partner and does battle with Inuyasha's group and was able to fend them off if only for a little while before fleeing. He later came across Hakudōshi and challenged him, but he seemed to be unable to even phase him. As the 'battle' dragged out, Entei began to second-guess Rengokuki's power and eventually thew him off his back, rejecting his old master and accepting Hakudōshi as its new master. Rengokuki was subsequently beheaded and Hakudōshi took his weapon as a prize.

From then on, Entei was ridden by Hakudōshi for all of his future appearances until he was finally vanquished by Inuyasha.

Personality
Entei does not seem to have any true loyalties and he aligns himself with the strongest demon he sees. In the anime, he does seem to show some loyalties to Hakudōshi as he covered his escape.

Powers & Abilities

 * Speed: Entei is infamous for his rather astounding speed and appears to be as fast as Sesshōmaru.
 * Fire: Being a fire elemental, Entei is able to breath fire from his mouth, spitting fire balls or a stream of fire, similar to a flamethrower.
 * Flame Burst: Entei used a powerful flame burst destroying the hill in which he was sealed.
 * Thunder: When concentrating, his demonic power can launch a very destructive and powerful electric attack.
 * Shōki: Entei can generate a poisonous miasma which he used to discourage Miroku from using his Kazaana.

Manga vs. Anime

 * The portrayal of Entei's death differs between the manga and the anime. In the manga, Inuyasha killed Entei and destroyed Hakudōshi's body with the Wind Scar simultaneously, after Saint Hijiri broke Hakudōshi's barrier. In the anime, the Wind Scar only hit Hakudōshi. Entei covered Hakudōshi's escape and engaged Inuyasha in combat, with Kirara carrying him on her back. The pair lured Entei to fly into a cave tunnel after them to cancel out his speed advantage before Inuyasha slew him with his Bakuryūha.

Trivia

 * It would seem that the fires on Entei's hooves won't extinguish when he's in water, just like the fire on Kirara's paws.
 * Entei shares its name with another fire-breathing creature, namely the Volcano Pokémon from the legendary beast trio of Generation II in the  games. Both have been said by legend to be born from an erupting volcano.
 * Also, Entei looks very similar to another 2 Pokémon: Ponyta and Rapidash.
 * In the opening One Day, One Dream, Kirara is shown to be able to keep up with Entei's speed.
 * The anime shows Shinsen sealing Entei and Rengokuki simultaneously even though both the anime and manga said he sealed them in two separate battles.

Media appearances
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Manga

 * Chapter 294
 * Chapter 295
 * Chapter 296
 * Chapter 298
 * Chapter 305
 * Chapter 306
 * Chapter 308
 * Chapter 322

Anime
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 * Episode 141
 * Episode 142
 * Episode 143
 * Episode 144
 * Episode 146
 * Episode 149
 * Episode 150
 * Episode 156