He was the Demon Bear dwelling in the forests. March was supposed the be the sacrifice to keep the bear away. He has many spikes in his body, which appeared to be spears. After he died, he became one of the forms of our main character.

A lonely boy wandering the Arctic regions of North America meets a wolf, and the two become fast friends, depending on each other to survive the harsh environment. But the boy has a history, and the wolf is more than meets the eye as well … 
 
(Source: Kodansha USA)

In the beginning, an "orb" is cast unto Earth. "It" can do two things: change into the form of the thing that stimulates "it"; and come back to life after death. "It" morphs from orb to rock, then to wolf, and finally to boy, but roams about like a newborn who knows nothing. As a boy, "it" becomes Fushi.
 
Through encounters with human kindness, Fushi not only gains survival skills but grows as a "person". But his journey is darkened by the inexplicable and destructive enemy Nokker and cruel partings with the people he loves.
 
(Source: Crunchyroll)

After seeing enough death and tragedy, the immortal Fushi secludes himself on an island, defending himself from enemy Nokkers. However, instead of attacking Fushi in isolation, Nokkers begin targeting the settlements outside of his reach in hopes of luring him out. Soon, a group known as the Guardians—led by Hisame, the descendant of the deceased warrior Hayase—finds Fushi.
 
Inspired by how Fushi protected Janada Island from the Nokkers years ago, the Guardians have grown a considerable following and are recognized throughout the world. Initially reluctant, Fushi allows the Guardians to accompany him to the site of the Nokkers' recent attack. In their village, Fushi meets a few valuable allies, both new and old. But as the conflict with the Nokkers only leads to more loss, Fushi must find the inner strength to face his inevitable sorrow.
 
(Source: MAL Rewrite)

Oniguma