Permanently missable Pokémon
There are several permanently missable Pokémon in the games. These are Pokémon species that the player may potentially obtain in the games, but may become permanently unavailable depending on the circumstances.
- Main article: Gift Pokémon
Most of the gift Pokémon are obtainable only once per game. Some of these Pokémon (as well as their evolutions and previous evolutionary stages, if any) are not available anywhere else in the game, except by trades or transfers from other games, or by breeding the gift Pokémon once it is obtained. Therefore, if the player loses any of these scarce gift Pokémon by either releasing it or trading it away without breeding it first, then it and its evolutionary family will not be available anywhere else in the current game.
For instance, in several Pokémon games, the only way to obtain a starter Pokémon of the current region (outside of trades and transfers from other games) is by choosing that starter at the beginning of the game. In some games (from Pokémon Emerald onwards), it also possible to obtain one or more starter Pokémon from other regions as gifts. If the player releases it or trades it away without breeding it first, then that starter and its evolutions would be unavailable in the current game.
As another example, in Pokémon FireRed and LeafGreen, the player is able to choose between receiving a Hitmonlee or a Hitmonchan. The player may then breed it to obtain Tyrogue, which can evolve into either of these Pokémon, as well as into Hitmontop. If the player obtains Hitmonlee or Hitmonchan but then releases it or trades it away without breeding it, then the whole evolutionary family (Tyrogue, Hitmonlee, Hitmonchan, and Hitmontop) would be unavailable in the current game.
Bulbasaur (Pokémon Yellow)
In Pokémon Yellow, the player may be permanently unable to receive the single available Bulbasaur. 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 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 (as it's not possible to change the friendship of a Raichu even if it returns to its original game), or if it was released in another game. Additionally, it can't be returned once it was sent to 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.
Wild Pokémon from in-game events
- Main article: List of wild Pokémon from in-game events
In most core series games, there is only one per species of Legendary Pokémon. In some cases, there is also a limited supply of other wild Pokémon from in-game events. For instance, there are only two Snorlax in Pokémon Red, Blue, Yellow, FireRed, and LeafGreen, and only one Sudowoodo in Pokémon Gold, Silver, and Crystal (this Sudowoodo respawns in Pokémon HeartGold and SoulSilver). Additionally, the player can breed several of these Pokémon to obtain more Pokémon of the same species. However, it's not possible to breed any Legendary Pokémon.
- In any game prior to Pokémon Platinum, if 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. Starting in Generation III, Legendary Pokémon respawn if the player runs, but other Pokémon can still become permanently unavailable.
- Additionally, even after the player obtains any of these Pokémon, the player may still lose it by either releasing it or trading it away.
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 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 also the only way to get an Electrode in a Beast Ball. Additionally, the only way to obtain Voltorb is to breed said Electrode with Ditto.
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.)
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.
In some games, there is a limited supply of certain Evolution stones or Incenses. If the player loses some of these items, then the respective Pokémon evolutions or pre-evolutions may be unavailable outside of trades.
The player may lose these items by selling or discarding them, or a Pokémon holding them may be traded away or released. Some moves (such as Fling and Trick) may also cause a Pokémon to lose its held item. Additionally, the Evolution stones are consumed when used to evolve a Pokémon; once all Evolution stones of a certain kind are used up, the player will be unable to evolve the respective Pokémon in the current game.
- There are five Moon Stones available in Pokémon Red, Blue, and Yellow, which are more than enough to evolve all the four compatible Pokémon in these games (Nidorina, Nidorino, Clefairy, and Jigglypuff into Nidoqueen, Nidoking, Clefable, and Wigglytuff, respectively). However, if the player evolves some repeated Pokémon of the same species (for instance by evolving five Jigglypuff into Wigglytuff), then the other evolutions would become unavailable in the current game.
- In Pokémon Gold and Silver, multiple Pokémon can evolve with a Fire Stone, Water Stone, Thunderstone, or Leaf Stone, but each of these stones is available only once per game (except from trades or Mystery Gift), so the player is unable to acquire all the available evolutions in a single game.
- In Pokémon Ruby, Sapphire, and Emerald, there is only one Sea Incense available. The only way to obtain Azurill outside of trades is by breeding Marill or Azumarill holding that item. Therefore, if the player loses that item, it would not be possible to obtain Azurill in the current game.
- 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).