You can exert Hydra Trainer as you declare it as an attacking creature. 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.
If you gain control of another player's permanent until end of turn and exert it, and then that player regains control of it, it will untap during that player's untap step.
You can always activate an ability that will cause a creature to adapt. As that ability resolves, if the creature has a +1/+1 counter on it for any reason, you simply won't put any +1/+1 counters on it.
Last observed on Scryfall 2 months ago at $0.23. Over the trailing 90 days the price moved between $0.03 and $0.24 across 49 observations. Cross-checked across TCGplayer, eBay sold, Cardmarket EU, and Star City.
This printing is one of 3 catalogued treatments from MH3 — the lineup includes Foil and Nonfoil alongside the standard finish. Across our catalog the Rarix library tracks every variant in the same family so collectors can compare price, scarcity and demand inside a single lineage.
Card #159 of *Modern Horizons 3*, the set that shipped alongside the June 2024 Modern Horizons 3 release.
Card type: Creature — Human Warrior, color identity green, printed at uncommon. Uncommons fill the middle-tier slots in a standard booster configuration. Mana cost: {1}{G} (2 converted). Keyword abilities: Adapt and Exert.
Legal across the major constructed formats — Modern, Legacy, Vintage, and Commander — plus Standard rotation play.
EDHrec ranks this card #11,452 in Commander, putting it in the fringe-play tier of the format's playable pool.
Illustrated by Ryan Pancoast. Rarix catalogs 890 other cards credited to this artist.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Ryan Pancoast
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
If an exerted permanent is already untapped during your next untap step (most likely because an effect untapped it), exert's effect preventing it from untapping expires without having done anything.
You don't choose a target for Hydra Trainer's first ability at the time it triggers. Rather, a second "reflexive" ability triggers when you exert Hydra Trainer this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.
The value of X is determined only once, as the reflexive triggered ability resolves.
If a creature somehow loses all of its +1/+1 counters, it can adapt again and get more +1/+1 counters.
