Cheapest priced copy (Innistrad: Double Feature): $0.35 at Card Kingdom, near mint, their price list as of .
Gryff Rider 
Creature — Human Knight · 2/1 · Common · Innistrad: Crimson Vow · #15
Flying
Training (Whenever this creature attacks with another creature with greater power, put a +1/+1 counter on this creature.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Innistrad: Double Feature | 282 | Common | $0.35 | $0.49 |
| Innistrad: Crimson Vow | 15 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 19 November 2021A creature's training ability triggers only when both that creature and a creature with greater power are declared as attackers. Increasing a creature's power after attackers are declared won't cause a training ability to trigger.
- 19 November 2021Once a creature's training ability has triggered, destroying the other attacking creature or reducing its power won't stop the creature with training from getting a +1/+1 counter.