Dog Residue
Dog Residue is an item left after the Annoying Dog absorbs the Waterfall artifact in Undertale.
How to get it
Solve the Waterfall piano puzzle, enter the artifact room, and let the Annoying Dog take the artifact. Dog Residue remains.
Why players keep it
Using it fills empty slots. Sell extras at Temmie Village for college tuition and Temmie Armor while keeping a single residue to regenerate more.