BIng Bong from Inside Out is a very happy character. He is Riley’s imaginary friend from when she was little. He is part cat, part elephant and part dolphin. It was extremely depressing when he sacrificed himself so Joy could escape the memory dump and set things right in Riley’s head. Before he died, he asked Joy to take Riley to the moon for him. Then that was it. Or was it? There’s more to Bing Bong than meets the eye.
Anyone who hasn’t read the Pixar Theory, it basically says that all Pixar films are in the same universe, but they all happen at different times. In the movie Monster’s Inc, we are shown that monsters use doors to get into children’s rooms and scare them to get scream power. But later they discovered that laugh is more powerful than scream and they began making children laugh. So if Inside Out is in the same universe as Monster’s Inc., then what if Bing Bong works for Monster’s Inc. and was Riley’s monster? He made her laugh to gather her laugh power and she liked Bing Bong so much, she made an imaginary friend based off him. Over the years Riley became too old for Bing Bong to entertain, so he stopped, resulting in him slowly being forgotten by Riley.
So even though Bing Bong dies in Inside Out as a mere memory of Riley, he is still out there somewhere making children laugh and giving them the gift of happiness.