Kalemne's Captain

Vigilance
{5}{W}{W}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
When Kalemne's Captain becomes monstrous, exile all artifacts and enchantments.

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Commander 2015

Vigilance{5}{W}{W}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
When Kalemne's Captain becomes monstrous, exile all artifacts and enchantments.

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.

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

Once a creature becomes monstrous, it can’t become monstrous again. If Kalemne’s Captain is already monstrous, the activated ability won’t do anything.