Fan Theory: Bing Bong Is a Monster From Monsters, Inc.
What's There
Bing Bong is Riley's forgotten imaginary friend from early childhood — part elephant, part cat, part dolphin, made partly of cotton candy — who lives inside her mind alongside her emotions.
Why It Matters
"The Pixar Theory" proposes that Bing Bong isn't purely imaginary at all, but a real monster from the world of Monsters, Inc. (2001) who once entered Riley's room to scare her, and instead made her laugh — explaining his character design (built from mismatched animal parts, the way many of that film's monsters are) and his eventual fate: since Monsters, Inc. establishes that laughter, not fear, is what powers the monster world, and Bing Bong sacrifices himself to save Riley's core memories, the theory frames his disappearance as thematically closing the loop between the two films rather than just being a sad character beat. This connects back to the Boo/Witch claim as another instance of the theory finding monster-world characters embedded in later, seemingly unrelated films. There's no on-screen confirmation of this — Bing Bong's design and backstory are original to Inside Out as far as any official source states.