The wolf is the pup Kalani saved at the climax
It's the only other wolf that was still living during Kalani's time, and her death imbued part of her soul into it. This is visually supported by the pup coming up to sleep with Atevo and Kalani's body, a clear parallel with the wolf's position between Kalani and Atevo's spirits. It being connected to them made it a perfect vehicle for teaching Atevo's spirit the lesson he needed.- Which begs the question of why it's still alive, given it's been long enough since Kalani and Atevo's day for most of their civilization's works to crumble.
The wolf was left behind when Kalani and Atevo went to the City of Light, but it will not be alone.
This is drawn from YouTube comments. Spirit wanderers have "one foot in this world and one foot in the other;" Kalani is part of the wolf but is not the wolf at the same time, and it still has its natural life to live out. It would not be fair to take it to the City of Light immediately. However, Kalani's not that cold, and part of her still stays with the wolf to give it the best life possible and retain the abilities it has learned to use. After all, the wolf still retains her marking.- Alternately, Wayla's spirit might escort the wolf to safety and the company of its own kind, to allow Atevo time to renew his bond with his daughter without his other victim's presence complicating matters, while repaying the wolf for helping Kilani resolve the regrets of her past.
