Trample doesn't knock you prone. In fact, trample requires that the targets be prone before you can even use it. At this point, it breaks limbs. A legendary steed can break all four at once.

The way this is pulled off by a geomancer is they can knock you prone (with a chance of breaking a leg)/stun at the same time, so the window of opportunity is a bit higher.

Luckily for druids paralysis counts as prone too (and Aquamancers can time jello/tsunami to go off together.)