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.
Hydra Trainer
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.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Ryan Pancoast
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
