Entei

Entei is a yokai horse that obeys only the most powerful yokai. Entei makes his first appearance in Chapter 294 "Entei" and in episode 141 named "Entei the Demon Horse Unleashed."

Background
The anime gives some insight into Entei's background which was completely unexplored in the manga. Entei first appears to a rather powerful naginata-wielding ogre demon who fought off hordes of demons with ease with a strong will. Recognizing his strength, Entei offers him his servitude and the two become a feared duo. Some time later, the demon is pacified by a powerful holy seal which is then destroyed by Hakoudoshi. Entei reunites with his old partner and does battle with Inuyasha's group and is able to fend them off if only for a little while before fleeing. He later comes across Hakudoshi and challenges him, but his seems to be unable to even phase him. As the 'battle' drags out, Entei begins to second-guess his current master's power and eventually throws him off his back, rejecting his old master and accepting Hakudoshi as its new master. The demon is subsquently beheaded and the offspring takes his weapon as a prize.

From forward on, Entei is ridden on horseback by Hakudoshi for all of his future appearances until he is finally vanquished by Inuyasha from his Backlash Wave.

Powers
Speed: Entei is infamous for his rather astounding speed and appears to be on the level's of Sesshomaru as he leaves afterimages in his wake when making short bursts

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

Etymology
It may have not been mentioned, but this uses the exact name and spelling of the Satoshi Tajiri's legendary Pokemon Entei, one of the fabled 'Legendary Dog Pokemon'.