The Prototype has an arm for every chapter (0_0)

Chapter 1: Huggy's arm

Chapter 2: Mommy's arm

Chapter 3: Catnap's fur

Chapter 4: Kissy's arm

What's for chapter five, I have a few guesses.

Either some of Lily's hair, or, the most promising one:

CHUM CHOMPKINS'S arm

After all, Kissy, Huggy, and Chum were all shown together that one time, and they already both lost an arm, so I think it's Chum's turn :(