From The Wajas Wiki

Gills are a type of Potion Mutation. They can be permanently added to your waja by using a Slime Potion. Gills give your waja three horizontal gills on each side of their neck, like a fish. When adding this mutation to your waja using a potion, you may choose their color.

Mutation Appearance

All examples feature wajas with FF0000 (pure red) gills. However, this mutation can come in any Hex Color as well as any Texture.