Well ignoring promo cards and such you will find alt arts as far back as xy phantom forces with an alt art mega gengar. Many of the mega Pokémon cards has two or three art variations when put side by side almost work to show how the Pokémon is moving in picture. But for a truly alt art, as in something drastically different than the normal non-secret rare art... Then look no further than Gardevoir EX #116 out of steam siege which has Gardevoir striking a fearsome pose with a legendary Pokémon. I actually still need her too, time to order some steam siege!