Cheapest priced copy (Commander 2015): $0.49 at Card Kingdom, near mint, their price list as of .
Arbor Colossus 
Creature — Giant · 6/6 · Rare · Commander 2015 · #174
Reach
{3}{G}{G}{G}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
When this creature becomes monstrous, destroy target creature with flying an opponent controls.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2015 | 174 | Rare | $0.49 | — |
| Theros | 150 | Rare | $0.49 | $0.49 |
3 rulings from Wizards of the Coast
- 15 September 2013An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.
- 15 September 2013Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
- 15 September 2013Monstrous 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.