Cheapest priced copy (Commander 2015): $0.35 at Card Kingdom, near mint, their price list as of .
Stoneshock Giant 
Creature — Giant · 5/4 · Uncommon · Theros · #142
{6}{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, creatures without flying your opponents control can't block this turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2015 | 165 | Uncommon | $0.35 | — |
| Commander Anthology Volume II | 120 | Uncommon | $0.35 | — |
| Conspiracy: Take the Crown | 169 | Uncommon | $0.35 | $0.49 |
| Theros | 142 | Uncommon | $0.35 | $0.49 |
5 rulings from Wizards of the Coast
- 15 September 2013If blockers have already been declared when Stoneshock Giant's last ability resolves, those blocks won't change or become undone.
- 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.
- 15 September 2013Your opponents can't assign any creature without flying to block that turn, even if that creature had flying or wasn't on the battlefield under one of your opponent's control when Stoneshock Giant's ability resolved.
- 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.