Permanently missable Pokémon
There are several permanently missable Pokémon in the games.
Wild Pokémon from in-game events
- Main article: List of wild Pokémon from in-game events
- In any game prior to Pokémon Platinum, the player defeats most of these Pokémon, they will not appear again.
- In Generations I and II, if the player runs from any of these Pokémon, it becomes permanently unavailable.
Additionally, there are Voltorb, Electrode, Foongus, and Amoonguss disguised as items in some games. They don't respawn if they are defeated. In most cases, other Pokémon of the same species are found elsewhere in the same games. However, in Pokémon Ultra Sun and Ultra Moon, the only Electrode available are the ones disguised as items in Team Rainbow Rocket's castle, so if they are defeated or skipped (as the area can't be revisited after defeating Giovanni), Electrode cannot be caught in these games. This is the only way to get an Electrode in a Beast Ball.
Mutually exclusive Pokémon
- Main article: Mutually exclusive Pokémon
In some cases, there are multiple Pokémon potentially available in a single game, but the player must choose one of them or otherwise can't catch all of them in a single game. They include many trios of starter Pokémon, some Fossil Pokémon and in some cases Legendary Pokémon.
For instance, the player is able to choose between Bulbasaur, Charmander, and Squirtle as their starter in Pokémon Red, Blue, FireRed and LeafGreen. The player may also choose one of these Pokémon as a gift given by Professor Oak in HeartGold, SoulSilver, by Professor Sycamore in X and Y, and by Grand Oak in Pokémon HOME.
- Main article: Event Pokémon
Some Pokémon, including the Mythical Pokémon, have been distributed in real-life events, as well as via Wi-Fi, Nintendo Network, or serial codes. Sometimes, the Pokémon were distributed directly, but at other times an item was distributed which leads to the respective Pokémon. However, these Pokémon and items have become unavailable once their respective events ended. Additionally, the real-life events were restricted to some regions, so they were not available anywhere else in the first place.
Usually, these Pokémon and items have been distributed only for some of the latest games, although some of the older Mythical Pokémon (such as Mew) have been repeatedly distributed in new events for each generation. When someone starts a new game on an older system such as the Game Boy or Game Boy Advance, they may not have access to any of the Pokémon from past events (except via trades or cheating).
Bulbasaur (Pokémon Yellow)
In Pokémon Yellow, the single Bulbasaur is permanently missable. To receive this Bulbasaur, the player's starter Pikachu needs to have a friendship value of at least 147. If the starter Pikachu is traded away or sent to the Pokémon Bank (from the Virtual Console version) before its friendship is raised to the required level, the player will be unable to receive Bulbasaur.
If the starter Pikachu is traded back to its original Pokémon Yellow game, its friendship can be raised normally. However, this may not be possible in some circumstances, such as if the starter Pikachu has evolved to Raichu, or if it was released in another game. Additionally, it can't be returned once it was sent to the Pokémon Bank.
Alternatively, even if the starter Pikachu is unavailable, if any other Pikachu with the same Original Trainer name and Trainer ID number is traded from any Generation I or II game, the traded Pikachu will be equally treated as Pokémon Yellow's starter Pikachu, and its friendship can be raised normally in order to receive Bulbasaur.
Roaming Roar bug (Pokémon FireRed and LeafGreen)
Due to an oversight in FireRed and LeafGreen, if the wild Raikou or Entei uses Roar, that Pokémon becomes permanently unavailable in the current game. (Suicune is unaffected by this glitch, because it does not have Roar in its moveset when found in these games.)