Berserking Rathtar

{2}{g}: Monstrosity 2 (If this creature isn't monstrous, put two +1/+1 counters on it and it becomes monstrous.)
When Berserking Rathtar becomes mostrous, all creatures able to block this turn do so.
"What's a rathtar?"
"You want the scientific description? They're big and dangerous and ugly."