So the abyssal is the tail...and the human body is...well, "former" human....I see. I've seen something similar in Bleach: 3rd Phantom game on Nintendo DS. One of the major characters transformed into an Arrancar-Shinigami hybrid, and he lost any human emotions---and to top it off, he also has a similar form to Re-Class's tail when he goes into his final form. The concept of abyssals are actually very similar to Hollows....including the fact that the more humanoid they are, the stronger they become.
Undoubtedly, it's a mutant.Found it kuma!!A core inside the tail's head!!...The power to regenerate in mere seconds even when the main body's core is destroyed.The two parts with same healing abilities, moving independently on their own...A "fusion" of two entities enabled by multiple cores.