"Ancient" (古Inishie) is an anime-only series used by Solomon Muto. Some of the cards are also owned by Solomon's friend, Arthur Hawkins. In the English dub it was said that Solomon made a bet where whoever found all seven cards in the set would get a free chocolate milkshake from the loser, which Solomon won.
The main strategy is to summon "Ancient Dragon" after the conditions are met. The Rock-Type "Ancient" monsters have the effect of inflicting damage to the player if they do not attack, but switching the positions of "Stone Giant" is crucial to meeting the summoning conditions of "Ancient Dragon".