Cheapest priced copy (Ravnica Allegiance): $0.35 at Card Kingdom, near mint, their price list as of .
Rampaging Rendhorn 
Creature — Beast · 4/4 · Common · Ravnica Allegiance · #135
Riot (This creature enters with your choice of a +1/+1 counter or haste.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ravnica Allegiance | 135 | Common | $0.35 | $0.49 |
| Ravnica Remastered | 154 | Common | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 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 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.