What is this! Fell off while driving

Probably part of the suspension. I think your car is a rust bucket. Time to have the car inspected.

Agreeing with F_O_R, looks like part of the stabilizer that was connected to the frame. Did the performance of your car change after that piece fell out?

Its a exhaust damper to stop the car from vibrating. And it passed inspection, good until june 2020. Shes over 15 years old. She’s gonna be rusty.

Ps. Everything on the underbody is not this rusted I promise

Thanks for the enlightenment. She looks like a pretty solid vehicle; after all, you're still driving her! Best wishes to you.

it stops harmonic distorsion in the car. some mount to transmission yours is mounted to exhaust flange.