The short film that will be paired with Cars 3 when it is released this upcoming summer is called, Lou, and thanks to USA Today we've got a first look at it (see picture above) and a few story details about it (see below).
Dave Mullins (an animator and supervising animator on many Pixar films) is directing the 6-minute short about a monster (the titular Lou) teaching a lesson to a bully called J.J.
Lou appears to be a creature composed of different toys in a Lost and found box; this and the 'teaching a bully some manners' may seem reminiscent of the good old Toy Story days, but Mullins' ideas seem very personal, and what he says about bullies is quite interesting:
"They’re usually just acting out because they’re awkward or young and don’t have their moral compass set. [...] If you can find out what their motivations are, maybe you can solve some things. That’s what I like about Lou: True happiness comes from giving."You'll be able to see for Lou in June if you live in the US, July if you're in the UK and on the Blu-Ray if you live in a country where they don't attach the Pixar short to the Pixar feature (as I do).