Stoneshock Giant

{6}{R}{R}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
When Stoneshock Giant becomes monstrous, creatures without flying your opponents control can't block this turn.

Pioneer Masters

Commander Anthology Volume II

Conspiracy: Take the Crown

Commander 2015

Theros

An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.

If blockers have already been declared when Stoneshock Giant's last ability resolves, those blocks won't change or become undone.

Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.

Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.

Your opponents can't assign any creature without flying to block that turn, even if that creature had flying or wasn't on the battlefield under one of your opponent's control when Stoneshock Giant's ability resolved.