Cheapest priced copy (Theros Beyond Death): $0.35 at Card Kingdom, near mint, their price list as of .
Stampede Rider 
Creature — Satyr · 2/3 · Common · Theros Beyond Death · #155
Trample
At the beginning of each combat, if you control a creature with power 4 or greater, this creature gets +1/+1 until end of turn.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Theros Beyond Death | 155 | Common | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 24 January 2020Once Stampede Rider’s ability has given it +1/+1, it won’t lose that bonus if you no longer control a creature with power 4 or greater later in the turn.
- 24 January 2020If you don’t control a creature with power 4 or greater as your combat phase begins, Stampede Rider’s ability doesn’t trigger. If you don’t control one as the ability resolves, it has no effect. It doesn’t have to be the same creature at both times, however.
- 24 January 2020Stampede Rider gets only +1/+1 from its ability, no matter how many creatures with power 4 or greater you control beyond the first.