Basic lands and cards printed at the Mythic Rare rarity are legal. This includes all cards which have at least one printing with a red-gold set symbol, even if the card itself is from a printing at a different rarity. The card does not need to have been available in a booster pack to be Mythic Rare; for example, cards which have only been printed at Mythic Rare in a supplemental product such as a Planeswalker Deck are still legal in Antipauper.