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.
Legalitet:
Standard: Ej legal
Pioneer: Ej legal
Modern: Ej legal
Legacy: Legal
Vintage: Legal
EDH / Commander: Legal
Pauper: Ej legal
Explorer: Ej legal
Historic: Ej legal
Brawl: Ej legal
Premodern: Ej legal
93/94: Ej legal