Cheapest priced copy (Ravnica Allegiance): $0.35 at Card Kingdom, near mint, their price list as of .
Gruul Beastmaster 
Creature — Human Shaman · 2/2 · Uncommon · Ravnica Allegiance · #129
Riot (This creature enters with your choice of a +1/+1 counter or haste.)
Whenever this creature attacks, another target creature you control gets +X/+0 until end of turn, where X is this creature's power.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ravnica Allegiance | 129 | Uncommon | $0.35 | $0.49 |
6 rulings from Wizards of the Coast
- 25 January 2019The value of X is determined only as Gruul Beastmaster’s triggered ability resolves. Once that happens, the value of X won’t change later in the turn even if Gruul Beastmaster’s power changes.
- 25 January 2019If you choose for the creature to gain haste, it gains haste indefinitely. It won’t lose it as the turn ends or as another player gains control of it.
- 25 January 2019If Gruul Beastmaster leaves the battlefield before its triggered ability resolves, use its power as it last existed on the battlefield to determine the value of X.
- 25 January 2019Riot is a replacement effect. Players can’t respond to your choice of +1/+1 counter or haste, and they can’t take actions while the creature is on the battlefield without one or the other.
- 25 January 2019If a creature entering the battlefield has riot but can’t have a +1/+1 counter put onto it, it gains haste.
- 25 January 2019If Gruul Beastmaster’s power is negative as its triggered ability resolves, X is considered to be 0. In other words, that ability can’t cause the creature to lose power.