Rolling Earthquake

• WWizards of the Coast2020-08-07
  Official Ruling
  Horsemanship is a keyword ability that functions similarly to flying. It appears in the Portal: Three Kingdoms set and does not appear in the Double Masters set. This means that in most cases Rolling Earthquake will simply deal X damage to each creature and each player, even creatures depicted on horseback in their illustration.

Rulings via Scryfall