Hydra Trainer

• WWizards of the Coast2024-06-07
  Official Ruling
  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.
• WWizards of the Coast2024-06-07
  Official Ruling
  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.
• WWizards of the Coast2024-06-07
  Official Ruling
  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.
• WWizards of the Coast2024-06-07
  Official Ruling
  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.
• WWizards of the Coast2024-06-07
  Official Ruling
  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.
• WWizards of the Coast2024-06-07
  Official Ruling
  The value of X is determined only once, as the reflexive triggered ability resolves.
• WWizards of the Coast2024-06-07
  Official Ruling
  If a creature somehow loses all of its +1/+1 counters, it can adapt again and get more +1/+1 counters.

Rulings via Scryfall