Cheapest priced copy (Commander Legends): $0.35 at Card Kingdom, near mint, their price list as of .
Ancient Animus 
Instant · Common · Dominaria · #154
Put a +1/+1 counter on target creature you control if it's legendary. Then it fights target creature an opponent controls. (Each deals damage equal to its power to the other.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Legends | 215 | Common | $0.35 | $0.49 |
| Dominaria | 154 | Common | $0.35 | $0.99 |
4 rulings from Wizards of the Coast
- 10 November 2020If the creature you control is an illegal target as Ancient Animus tries to resolve, you won't put a +1/+1 counter on it. If that creature is a legal target but the creature you don't control isn't, you'll still put the counter on the creature you control if it's legendary.
- 10 November 2020If either target is an illegal target as Ancient Animus tries to resolve, neither creature will deal or be dealt damage.
- 10 November 2020You can't cast Ancient Animus unless you choose both a creature you control and a creature you don't control as targets.
- 10 November 2020The creature you control doesn't have to be legendary. It simply won't receive a +1/+1 counter before it fights if it isn't legendary.