Ryūkotsusei

Ryukotsusei (竜骨精 Ryūkotsusei) was a dragon daiyōkai who fought with Inu no Taishou long before the main storyline. He never was able to fully kill Ryukotsusei but managed to seal him into a dormant sleep with one of his claws. Soon after this battle however, Inu no Taishou succumbed to his injuries and died.

After Tetsusaiga was destroyed by Goshinki, Inuyasha had to get it reforged using one of his own fangs. Unfortunately for Inuyasha though, Tetsusaiga became much heavier than before and became harder for him to wield. After seeking an explanation from Totosai and Myoga, he goes off stubbornly, to kill Ryukotsusei. Myoga explained to Inuyasha that he only had to strike Ryukotsusei in the heart and he would've surpassed his father however, Inuyasha disagreed with this method saying that it wasn't his style. Fortunately for Inuyasha, Naraku had removed Inu no Taishou's claw, reawakening Ryukotsusei, in hopes that he would be able to destroy Inuyasha.

Appearance
Ryukotsusei has the appearance of a typical dragon but with a mask on his head. This mask is Ryukotsusei's real face as it does the talking and directs the body while the dragon's mouth shoots out powerful energy blasts. Both the mask and the dragon have red eyes. Its long body is covered in thick armored hide which is near impenetrable as Inuyasha quickly realizes. He has two fairly short arms however he was able to slam Inuyasha up against the cliffside without struggle. Along with his large intimidating body, his demonic aura can easily disintegrate rocks without even having the dragon come in contact with it. Unlike Inu no Taishou, Sesshomaru, or Inuyasha, Ryukotsusei does not appear to have a humanoid appearance.

Reawakening
With the claw removed, Ryukotsusei awoke to see Inuyasha. They immediately engaged into a fight with Ryukotsusei on the offensive. Unbeknownst to Inuyasha, Ryukotsusei was aiming for Naraku's demon puppet before truly turning his attention to him. He learns that Inu no Taishou had died from the very battle that had sealed him and that Inuyasha was his son. Ryukotsusei constantly looked down at Inuyasha calling him a 'little boy' and ridiculing Tetsusaiga on its effectiveness against him.

Enhanced Strength
Ryukotsusei was able to dominate over Inuyasha for the first half of the battle, only to momentarily overwhelmed by his demonic transformation. His full power was not shown in the series, but it was enough for one of the strongest characters in the series, the Great Dog Demon,to be unable to defeat him and had to seal him away, and is the ultimate reason for his death; suggesting he was very formidable. It's implied that he was still recovering from the injury caused by Inuyasha's father and wasn't at full strength, and likely lost to this and his undermining of his last opponent, in addition to the accidental use of the Bakryuha that killed him.

Enhanced Defense
Ryukotsusei has an incredible armored hide that is nearly impenetrable, even striking the scar Inu no Taishou's claw left in it had no effect. However, when Inuyasha transformed, he was able to inflict wounds on Ryukotsusei with his claws alone. After Inuyasha repossessed Tetsusaiga, he was able to stab Ryukotsusei in the heart and use the Wind Scar on him; however Ryukotsusei did not die despite these attacks.

Thunder Attack Bullet
These large, powerful, pure white energy spheres are launched out of Ryukotsusei's mouth and can be fired numerous times consecutively. In the anime, it was so powerful that when Miroku used his Wind Tunnel on one of these blasts, he was only able to change its course slightly. Tetsusaiga's sheathe was able to protect Inuyasha from one of these attacks.

Death
Inuyasha managed to kill Ryukotsusei by using the Bakryuha (without realizing it) on one of Ryukotsusei's Thunder Bullets which tore the demon into pieces. From Ryukotsusei's death, Inuyasha has "surpassed" his father, in that he no longer needed to rely on his father's power to protect him. Sesshomaru would later follow in these same footsteps, upon forming the Bakusaiga.

Name Meaning
Ryukotsusei literally means "dragon bone spirit".