Event Pokémon are Pokémon that are distributed at promotional events, such as movie premieres, expositions, festivals, etc., and sometimes found in bonus discs or specially transferred from another game.
From the very beginning, there were Pokémon that were made not obtainable in the initially-released pair of games, making these species of Pokémon only legitimately obtainable from Nintendo's promotional event distributions. In later generations, some of these event-exclusive Pokémon became available through spin-off Pokémon games. However, besides these games, these Pokémon can only be obtained from events, or for some, they can only be caught in the games after an in-game event is unlocked through a real-world event.