A superior type of changeling, exemplified by Juice Box's brother, who collected touched forms into a personal library.
Recalling Juice Box's brother, who died on the fifth floor, Carl notes he was a type of Changeling called a changeling principal. Like all changelings he could only assume forms he had personally touched, and he had turned that into a quest: while working to get his people off the fourth floor, he also set out to touch as many rare creatures as he could in order to fill his "library" of available shapes.
The distinction matters to Carl only because it sharpens the puzzle. If even a principal has to touch a thing to wear it, then Juice Box's ability to wear The Beautiful Place — a creature that has never been in the dungeon — should not be possible.
An elevated changeling variant whose form-library mechanic frames the mystery of Juice Box's impersonation.
Also appears in books you've already read — spoiler-safe to their endings: