The Pixar Theory is an idea forwarded by Jon Negroni that all of Pixar’s movies take place in the same universe and are all connected to each other somehow. (Negroni turned the theory into a 100-page book.)
Centuries later, the animals from Brave that have been experimented on by the witch have interbred, creating a large-scale population of animals slowly gaining personification and intelligence on their own.
There are two progressions: the progression of the animals and the progression of artificial intelligence. The events of the following movies set up a power struggle between humans, animals, and machines.
The stage for all-out war in regards to animals is set by Ratatouille, Finding Nemo, and Up, in that order. Notice I left out A Bug’s Life, but I’ll explain why later.
Last week, the official Toy Story account released a video on Facebook that make explicit many of the connections between the films:
One of the dinosaurs from The Good Dinosaur shows up in Inside Out, a Monsters Inc. character is pictured in Brave, a Lightning McQueen toy is in Toy Story 3, a moped from Ratatouille is in Wall-E’s junkyard, etc. etc. This is a perfect bit of superfan trolling from the Pixar team. Kudos.