Whenever Pugnacious Hammerskull attacks while you don’t control another Dinosaur, put a stun counter on it. (If a permanent with a stun counter would become untapped, remove one from it instead.)
If you didn't control another Dinosaur when you declared Pugnacious Hammerskull as an attacker, it doesn't matter whether or not you control one as its ability resolves. You'll still put a stun counter on Pugnacious Hammerskull.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.