Cheapest priced copy (Modern Horizons): $0.35 at Card Kingdom, near mint, their price list as of .
Bellowing Elk 
Creature — Elk · 4/2 · Common · Modern Horizons · #157
As long as you had another creature enter the battlefield under your control this turn, this creature has trample and indestructible.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Modern Horizons | 157 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 14 June 2019Bellowing Elk’s effect checks each turn whether it applies during that turn. It doesn’t check only during the turn it enters the battlefield.
- 14 June 2019Bellowing Elk’s effect applies even if the other creature entered the battlefield under your control before Bellowing Elk entered the battlefield, and it continues to apply even if that creature leaves the battlefield.