Rengokuki

was the original demon who was chosen by Entei as its rider.

History
Rengokuki was one among countless humans and demons who set out to prove their worthiness as rider of the fearsome demon horse Entei, as legend held that he who rode it would rule the world. Rengokuki was the sole survivor of this tournament, and was allowed to ride astride the beast. Some time following these battles, Rengokuki, along with Entei, were sealed away by a powerful priest named Shinsen, and remained so for several decades until the priest was killed by the Infant.

During the story
Released from his seal by Entei, Rengokuki attempted to resume terrorizing villages and conquering the world. He impulsively offered to make Kagura his mate (in Japanese, he asked her if she thought he was impressive before offering to be his bride), but she had no interest in him whatsoever, saying that she didn't think he looked like he could take over the world. After Kagura rejected him, Rengokuki chased after her on Entei in anger and attempted to kill her for insulting him. But before they were able to battle one another, Hakudōshi appeared and said he'd take care of Rengokuki. Rengokuki scoffed at the child, thinking he was no match for him at first glance. Entei then threw Rengokuki off of his back, and stared transfixed at Hakudōshi. This action shocked and stung Rengokuki, who then vowed to destroy the boy to prove to Entei that he was its true rider. Rengokuki swung his halberd at Hakudōshi, but missed him completely; Hakudōshi stood on the tip of the weapon, much to Rengokuki's astonishment. Hakudōshi then easily tore the halberd out of Rengokuki's grip and brutally ran it through his throat, then vertically bisected his upper half with it, killing him. Following his death, Rengokuki was usurped by Hakudōshi as Entei's new master.

Personality
Rengokuki was an arrogant and overconfident individual. Having won many battles to win Entei's favor gave him a high sense of self-worth and after earning the right to ride Entei, Rengokuki considered himself unbeatable. It was his pride as a fighter that led indirectly to his undoing, as Kagura's refusal to marry him and his desire to punish her for this slight led to his encounter with Hakudōshi.

Nevertheless, his constant need to prove his worthiness to Entei as well as others indicates that he was insecure and desperate to retain control of his steed. When Inuyasha overpowered his energy blasts, Rengokuki commented to Entei that his weakness stemmed from hunger and that there was no other way he could have lost to Inuyasha. Despite being rejected by Entei in favor of Hakudōshi, Rengokuki still underestimated his foe, and his desire to regain Entei's esteem led him to rush carelessly into battle. The combination of these two factors made him an easy target for Hakudōshi and hastened his demise.

Physical description
Rengokuki had dark green skin with hay-colored hair. He also wore armor and had a short horn sticking out of his forehead.

Powers & Abilities
Rengokuki was a reasonably powerful demon. He could also fire energy blasts from his mouth. They had enough energy to knock Inuyasha back when he blocked them with Tessaiga, and set fire to various houses and even hydrated grass.

Weapons

 * Naginata: Rengokuki was in possession of a naginata. He was skilled with a it. He was killed by his own weapon at the hands of Hakudōshi. It became Hakudōshi's weapon of choice since then.

Relationships
Rengokuki won countless battles to win Entei's favor, but still felt the urge to constantly please it and make excuses for his shortcomings. Though he would often claim he was the owner of the fearsome demon horse, the fearsome demon horse owned him.
 * Entei

Manga vs. Anime

 * Rengokuki was a character exclusive to the anime, created most likely to give viewers an idea of just how strong Hakudōshi was. In the manga, Entei chose Hakudōshi as its rider because he killed the priest that sealed him.
 * Hakudōshi obtained his naginata from Rengokuki after killing him in the anime. In the manga, he obtained his naginata from a boar yōkai that looked very similar to another anime-exclusive character Chokyūkai.