Fishlegs' Dragon Cards are like the Dragon Eye Lenses, as they tell him and his friends about the "new" unknown dragons they've come across; he keeps them in the pouch on his belt. They each have an image of a dragon, along with their class, size, speed, abilities, strengths and other important information.
They were first introduced in the second film.
In "Crash Course", when the Riders came across the Cavern Crasher on Fireworm Island, Fishlegs got the cards out of his pouch to identify the dragon. Fishlegs clamed that he had been working on them for months, before telling his friends how they work. The Cavern Crasher Card gave them an idea why it was attacking the Fireworm Queen's nest, as well as the dragon's ability's, class and speed.
During the events of How to Train Your Dragon 2, after Eret brought Astrid and the other Riders to Drago's location, they saw a group of ships around a "pool" of bubbling bubbles. Fishlegs got his cards out to see if those bubbles were made by dragon and what kind it is, he had trouble finding the card that can tell him what dragon it is and when Drago's men "found" them, Fishlegs dropped the cards in his hands when he put to surrender. It's most likely that he came back for the cards or picked them up, as he has them during the events of "The Serpent's Heir".
- The cards and the pouch their kept in appears in the game Dragons: Rise of Berk as a collectable in completing collections.
- Its possible that Fishlegs had got to work them after the Dragon Eye was taken by the Dragon Hunters, and that some of the card's information are from some of his Dragon Eye notes.