Stormbreath Dragon

{3}{R}{R}Creature — Dragon4/4

Flying, haste, protection from white

{5}{R}{R}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)

When this creature becomes monstrous, it deals damage to each opponent equal to the number of cards in that player's hand.

Flying. This creature can't be blocked except by creatures with flying or reach. · CR 702.9

Haste. This creature can attack and use tap abilities as soon as it comes under your control. · CR 702.10

Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens. (2013-09-15)
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. (2013-09-15)
For each opponent, count the number of cards in that player's hand when the last ability resolves to determine how much damage Stormbreath Dragon does to them. (2013-09-15)
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. (2013-09-15)