Cheapest priced copy (Amonkhet): $0.35 at Card Kingdom, near mint, their price list as of .
Trueheart Twins 
Creature — Jackal Warrior · 4/4 · Uncommon · Amonkhet · #153
You may exert this creature as it attacks. (It won't untap during your next untap step.)
Whenever you exert a creature, creatures you control get +1/+0 until end of turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Amonkhet | 153 | Uncommon | $0.35 | $0.49 |
| Foundations Jumpstart | 605 | Uncommon | $0.35 | — |
5 rulings from Wizards of the Coast
- 18 April 2017If you gain control of another player's creature until end of turn and exert it, it will untap during that player's untap step.
- 18 April 2017Some cards have abilities that trigger whenever you exert any creature. These abilities trigger when you exert that creature or any other creature you control.
- 18 April 2017All cards in the Amonkhet set that let you exert a creature let you do so as you declare it as an attacking creature, as do some of the cards in the Hour of Devastation set. You can't do so later in combat, and creatures put onto the battlefield attacking can't be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.
- 18 April 2017If an exerted creature is already untapped during your next untap step (most likely because it had vigilance or an effect untapped it), exert's effect preventing it from untapping expires without having done anything.
- 18 April 2017You can't exert a creature unless an effect allows you to do so. Similar effects that "tap and freeze" a creature (such as that of Decision Paralysis) don't exert that creature.