This page of the Horizon Forbidden West guide contains information on how to distract enemies with thrown rocks.
Rocks - where to look?
Rocks in Horizon Forbidden West can be found quite literally anywhere, but you don't need to collect too many of them as they are reusable and can be picked up again after throwing them.
Rocks - how to throw?
First make sure you have the rocks selected as the Down item on the D-Pad. If not, you can freely switch between them by pressing left or right on the D-Pad.
Press Down to make the throw's trajectory appear on the screen. Aim carefully, then press R2 to throw the rock, as if you were shooting a bow.