"I don't know whether it hates the mycoids or just loves breaking things. I say we leave it be."
—Samin, Oltec scout

Pugnacious Hammerskull

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.)

The Lost Caverns of Ixalan Promos

The Lost Caverns of Ixalan

Whenever Pugnacious Hammerskull attacks while you don't control another Dinosaur, put a stun counter on it.

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.