(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
In testing, we found Diminishing Returns difficult to resolve at four mana, and even if you did so you were often tapped out even in the late game due to a flurry of instants being cast with it on the stack
It's got X kicker not 0 though
In any vampire deck, a turn 1 Blade of the Bloodchief spells doom for your opponents, and there are a couple infinites you can pull off with the Blade's counter production
If you think of this ability as tapping to make a vanilla token, it sounds pretty good -- and that's basically what it is, even if it does take the extra step of having something in your graveyard