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Terrakion, a Secret card in both English and Japanese
A secret card is a card whose collection number is greater than the listed number of cards in the set, for example, numbered 101/100. Secret cards are generally rarer than the other cards in the set, and typically are holofoil cards as well.
The first of the secret cards which appeared was Dark Raichu in the Team Rocket expansion. It was not included even in the Japanese version of the set, as it was made by Wizards itself.
Prior to the Black & White era, Secret cards were almost exclusively found in English versions of the sets, as nearly all cards released as "secret" in English have been included in the normal set numbering in Japanese or were featured as promo cards, with the exception of the Japanese sets Wind from the Sea, Split Earth and Mysterious Mountains. Interestingly, all Japanese Full Art cards are Secret cards, while in the English sets, they are included in the normal set numbering.
Many secret cards are also Box Toppers.
List of secret cards