The idea of Waka was born in a tabletop role game that a few friends and I used to play long time ago c:

Waka is an oily dog that has no brain, feelings, and can't feel any pain or love. They are only driven by his primary necessity: food, wandering around the world without direction, eating spirits and/or magic things or beings.

Their oily/gooey nature makes him almost unaffected by any phisical damage. They consume any magic that's nearby. Still, their body is really susceptible to any kind of anti-magic damage, shields, seals or skills. Their core is more resistant, nevertheless.

The ring around their snout is a seal to prevent them to over consume themelves. This seal repairs itself each time they finish a meal, and breaks apart each time they're craving for food.
If Waka can't find a reliable source of food for a long time, they'll start eating themselves and try to eat living things, like plants, trees and animals.