Caustic Bronco

• WOTC Official Reminder Textofficial rules
  Saddle — Tap any number of other untapped creatures you control with total power N or greater: This Mount becomes saddled until end of turn. Saddle only as a sorcery.

• WWizards of the Coast2024-04-12
  Official Ruling
  “Saddle N” means “Tap any number of other untapped creatures you control with total power N or greater: This permanent becomes saddled until end of turn. Activate only as a sorcery.”
• WWizards of the Coast2024-04-12
  Official Ruling
  If a permanent becomes a copy of a saddled Mount, the copy won’t be saddled.
• WWizards of the Coast2024-04-12
  Official Ruling
  You may activate a permanent’s saddle ability even if that permanent is already saddled.
• WWizards of the Coast2024-04-12
  Official Ruling
  “Saddled” isn’t an ability that a creature has. It’s just something true about that creature. It won’t stop being saddled until the turn ends or it leaves the battlefield.
• WWizards of the Coast2024-04-12
  Official Ruling
  Creatures with saddle can attack or block as normal even if they aren’t saddled.
• WWizards of the Coast2024-04-12
  Official Ruling
  If the revealed card doesn’t have a mana cost (because it’s a land card, for example), its mana value is 0.
• WWizards of the Coast2024-04-12
  Official Ruling
  If the revealed card is a split card, that card’s mana value is based on the combined mana cost of its two halves.
• WWizards of the Coast2024-04-12
  Official Ruling
  If Caustic Bronco isn’t on the battlefield as its triggered ability resolves, use whether it was saddled or not before it left the battlefield to determine who loses life.
• WWizards of the Coast2024-04-12
  Official Ruling
  An ability that triggers when a creature “attacks while saddled” will trigger only if that creature was saddled when it was declared as an attacker.
• WWizards of the Coast2024-04-12
  Official Ruling
  If the revealed card’s mana cost includes , X is 0 for the purpose of determining its mana value.

Rulings via Scryfall