There are 6 Common cards, 1 Common or The List card (The List being special cards, usually reprints, the selection of which can change between sets), three Uncommon cards, 1 non-foil Wild Card (this can be anything from the main set that the booster is a part of), 1 Rare or Mythic Rare from the boosters set, 1 Foil card of any rarity, 1 Basic Land card and, finally, 1 non-playable card, which can be a token, play aide, ad card or art card
It works similar to legendary in how it can apply to several different card types permanents and nonpermanents
Pros: Can reliably access it via companion ability Great in ETB strategies Can cheat mana costs of other creatures with its ability Cons: Not especially powerful if its ability misses How you can get it: Ikorai: Lair of Behemoths 2
Card not found was a major player in Standard for a while and has made appearances in pretty much all Constructed formats, including some play in Vintage