Mouse Trapper

• WOTC Official Reminder Textofficial rules
  Valiant — This ability triggers whenever this creature becomes the target of a spell or ability you control.
• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2024-07-26
  Official Ruling
  If a spell or ability you control has one or more of its targets changed to a creature you control with a valiant ability, that ability will trigger if that creature hasn't yet been the target of a spell or ability you control this turn.
• WWizards of the Coast2024-07-26
  Official Ruling
  Valiant abilities will resolve before the spell or ability that caused them to trigger.
• WWizards of the Coast2024-07-26
  Official Ruling
  If you create a copy of a spell on the stack and target a creature you control with a valiant ability, that ability will trigger as long as the creature hasn't yet been the target of a spell or ability you control this turn.

Rulings via Scryfall