This fell out of my brakes, what is it and where does it go? 2018 Prius C.

i haven't taken mine far enough apart to have one of those fall out yet, but it looks about right for one of the doodads that pushes the rearward brake shoe and scissor bar when you yank the parking brake handle.

 

Drum brakes usually have a self adjuster at the bottom that keeps the shoes properly positioned as they wear. Each end of that self adjuster has a small fitting that engages on the shoe base to lock everything in place. It looks similar to one of those pieces. Look at the bottom of this generic drawing at the part labeled adjuster assembly. On the right of that assembly is the piece that looks like your doodad, and the 'fork' section slips onto the brake base.

 

Adjuster bar is just under the wheel cylinder- and the 'claws' it has to push the shoes are much thinner and longer.

That's why I think this is a 'claw' from the parking brake, not visible behind bearing hub plate in this photo.

 

May be late but here's the installation for the rear drum brake from TIS. Same configuration for a 2015, cannot comment for the newer 2018