Mega Legendary cards are created by fusion. There currently is no mention of them in the wiki.
There are 73 uncommon cards, 89 rare cards, 60 epic cards, and 42 legendary cards you can collect. Cards are classified as Monsters or Stones. Total: 264 collectible cards. Some monster cards can evolve, so there are actually more than 264 unique cards. Exact number to be determined later. Aspect cards are cards with extra skin(s) with no actual change to their stats or abilities. It is purely decorative.
There are ten elements in the game, Mystic, Fire, Water, Electric, Earth, Plant, Wind, Dragon, Mech, and Rainbow. All monsters belong to a specific element, except rainbow. Stones may be any of the elements, including rainbow. Stones can be divided into evolution or utility. All cards, monsters and stones, have five possible rarity, Starter, Uncommon, Rare, Epic, and Legendary.
Most cards belong to a generation. Currently there are two generations, Core and Origin. All rewards (mostly eggs and packs) can only belong to a specific generation. For example, the Mystery Pack for 100 gold is of the Core generation, and the Origin Pack for 150 gold is of the Origin generation. Cards that do not belong to a generation usually are one-time rewards or tournament fragments, and such are classified as Special generation.