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Monstrosity“Monstrosity N” means “If this permanent isn’t monstrous, put N +1/+1 counters on it and it becomes monstrous.”
Monstrous is a designation that has no rules meaning other than to act as a marker that the monstrosity action and other spells and abilities can identify. Only permanents can be or become monstrous. Once a permanent becomes monstrous, it stays monstrous until it leaves the battlefield. Monstrous is neither an ability nor part of the permanent’s copiable values.
If a permanent’s ability instructs a player to “monstrosity X,” other abilities of that permanent may also refer to X. The value of X in those abilities is equal to the value of X as that permanent became monstrous. (CR 701.37) 1. (If this creature isn't
monstrousA designation a permanent gains from monstrosity until it leaves the battlefield. (CR 701.37), put a +1/+1
counterTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) on it and it becomes
monstrousA designation a permanent gains from monstrosity until it leaves the battlefield. (CR 701.37).)
When this creature becomes
monstrousA designation a permanent gains from monstrosity until it leaves the battlefield. (CR 701.37), it deals 2 damage to each opponent and each creature they control.