Champion of Rhonas

All cards in the Amonkhet set that let you exert a creature let you do so as you declare it as an attacking creature, as do some of the cards in the Hour of Devastation set. You can’t do so later in combat, and creatures put onto the battlefield attacking can’t be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.

If the creature put onto the battlefield has any abilities that trigger when creatures attack or when you exert creatures, those abilities won’t trigger.