Punishing Tail A creature struck by the ankylosaurus's tail must attempt a DC 24 Fortitude save.
On a failure, it's Stunned 1; on a critical failure, it's Stunned 3.
Trample◆◆◆ Medium or smaller, foot, DC 24 basic Reflex
Source: Pathfinder Monster Core
Running this fight? Add the ankylosaurus to the initiative tracker: type its name and the AC, hit points and Perception for initiative fill in, and the book button shows this stat block mid-fight.