Inside Out Theory
Theory: Bing Bong from Inside Out isn’t dead, but actually a monster from Monsters, Inc. who Riley thought was imaginary.
At first look, it seems like Riley’s imaginary friend, Bing Bong, sacrifices himself in Inside Out so that Joy can remain with Riley, but this Reddit theory poses that he wasn’t an imaginary friend at all, but instead a monster from Monsters, Inc. Bing Bong entertains Riley when she is a child, just like Mike and Sully did with Boo, but when Riley grows up, she makes the assumption that the monster was something she had imagined when she was small.
It fits in with the previously mentioned Pixar theory and further explains the monster energy crisis in Monsters, Inc. One Redditor says, “The way Inside Out has had an effect on the monsters’ world is that Inside Out showed us that emotions are living entities and create physical memories. And if the monsters have in fact been collecting memories, or rather, emotions, then that would explain their energy crisis.”