Since the last couple of Generations, the Pokemon Developers have used a New Generation to Pokemon as a chance to make many readjustments. When the Sixth Generation came, many older Pokemon had their abilities shuffled or an entirely new one has been added. And so the same has now occurred with the Seventh Generation.
Updated Abilities For Older Pokemon
Gengar: Levitate --> Cursed Body
Raikou: Volt Absorb --> Inner Focus
Entei: Flash Fire --> Inner Focus
Suicune: Water Absorb --> Inner Focus
Wingull: Hydration Added
Pelipper: Drizzle Added
Torkoal: Drought Added
Roggenrola: Weak Armor Added
Boldore: Weak Armor Added
Gigalith: Sand Stream Added
Vanillite: Snow Cloak Added
Vanillish: Snow Cloak Added
Vanilluxe: Snow Warning Added
Cubchoo: Slush Rush Added
Beartic: Slush Rush Added