# Blue #

## Red/Blue ##

Pidgeot knows Sky Attack, which is learned through a TM that can only be obtained once.

Alakazam is a trade evolution, requiring a second game to obtain.

Rhydon evolves from Rhyhorn, which is only available in the Safari Zone.

**If your chosen starter was Bulbasaur or Charmander:** Exeggutor evolves from Exeggcute, which is only available in the Safari Zone.
Getting its moveset may also be potentially annoying, as you must catch it at Level 24 to prevent it from knowing Reflect, and then evolve it before leveling it up to teach it Stomp.

**If your chosen starter was Charmander or Squirtle:** Arcanine evolves from Growlithe, which is a version exclusive Pokemon not available in Blue.

**If your chosen starter was Bulbasaur or Squirtle:** Gyarados knows Dragon Rage, Hydro Pump, and Leer, which are learned at Level 1 in Red/Blue. A Gyarados cannot be legitimately obtained in Red/Blue that can learn these moves, and there is no move relearner in this generation, meaning his Gyarados must be traded from Yellow, where one can be caught at Level 15.

Blue uses a starter you do not have access to, requiring a trade. (I didn't check movesets, those may be additional hurdles.)

## Yellow ##
See [this question][1] for info on how Blue's starter evolutions are determined.

Sandslash knows Earthquake, which is learned through a TM that can only be obtained once.

Alakazam is a trade evolution, requiring a second game to obtain.

Exeggutor evolves from Exeggcute, which is only obtainable in the Safari Zone.

**If Blue's Eevee evolved into Vaporeon or Jolteon:** Ninetales evolves from Vulpix, which is only obtainable in the Game Corner.

**If Blue's Eevee evolved into Vaporeon or Flareon:** Magneton knows Thunderbolt, which is learned through a TM that can only be obtained once.

**If Blue's Eevee evolved into Jolteon or Flareon:** Cloyster knows Ice Beam, which is learned through a TM that can only be obtained once.

**If Blue's Eevee evolved into Jolteon:** Jolteon knows Thunderbolt, which is learned through a TM that can only be obtained once.

## FireRed/LeafGreen ##

Pidgeot knows Aerial Ace, which is learned through a TM that can only be obtained once.

Alakazam...

- is a trade evolution, requiring a second game to obtain.
- knows Psychic, which is learned through a TM that can only be obtained once.

Rhydon knows Rock Tomb, which is learned through a TM that can only be obtained once.

**If your chosen starter was Bulbasaur or Charmander:** Exeggutor knows Giga Drain and Light Screen, which are both learned through TMs that can only be obtained once.

**If your chosen starter was Charmander or Squirtle:** Arcanine evolves from Growlithe, which is a version exclusive Pokemon to FireRed and XD: Gale of Darkness.

Blue uses a starter you do not have access to, requiring a trade. (I didn't check movesets, those may be additional hurdles.)

# Lance #

## Generation II ##

Charizard is only obtainable by transferring from a Generation I game.

Aerodactyl knows Rock Slide, which it cannot learn legitimately.

All of Lance's Dragonite are lower level than what is legitimately obtainable.

Lance's Level 50 Dragonite knows Outrage, which it cannot learn legitimately at this level.

## HeartGold/SoulSilver ##

Gyarados...

- knows Dragon Pulse, which is learned through a TM that can only be obtained once before post-game.
- knows Flail, which it cannot learn legitimately.

Charizard is only obtainable in post-game.

Aerodactyl...

- is only obtainable in post-game.
- knows Rock Slide, which is learned though a TM only obtainable in post-game.
- knows Aerial Ace, which is learned through a TM that can only be obtained once before post-game.

All of Lance's Dragonite are lower level than is legitimately obtainable.

Lance's Level 50 Dragonite knows Outrage, which is only learnable at this level through either the Battle Frontier or an event distribution.

# Steven Stone #

## Ruby/Sapphire ##

Skarmory knows Toxic and Aerial Ace, which are both learned through TMs that can only be obtained once.

Claydol knows Earthquake, which is learned through a TM that can only be obtained once.

Aggron...

- knows Earthquake, which is learned through a TM that can only be obtained once. Since Claydol also knows Earthquake, you must either breed to pass the move down or trade the TM from another game.
- knows Dragon Claw and Solar Beam, which are both learned through TMs that can only be obtained once.

You can only obtain one of the fossil Pokemon per save file, requiring a trade.

Cradily knows Sludge Bomb and Giga Drain, which are both learned through TMs that can only be obtained once.

Armaldo...

- knows Aerial Ace, which is learned through a TM that can only be obtained once. Since Skarmory also knows Earthquake, you must either breed to pass the move down or trade the TM from another game.
- knows Water Pulse, which is learned through a TM that can only be obtained once.

Metagross...

- is only obtainable in post-game. (This is not a very big issue, as Steven does not have a rematch team in this game.)
- knows Earthquake, which is learned through a TM that can only be obtained once. Since Claydol and Aggron also know Earthquake, you must either breed to pass the move down or trade the TM from another game.

## Omega Ruby / Alpha Sapphire ##

- You can only obtain one of the fossil Pokemon per save file, requiring a trade.

- Armaldo knows Rock Blast, which cannot be learned at its current level without trading with XY.

- Metagross is only obtainable post-Delta Episode.

# Wallace #

Wailord...

- knows Rain Dance, which is learned through a TM that can only be obtained once.
- knows Double-Edge, which is learned through either a move tutor or breeding.

Tentacruel knows Toxic and Sludge Bomb, which are both learned through TMs that can only be obtained once.

Gyarados knows Earthquake, which is learned through a TM that can only be obtained once.

Milotic...

- is a difficult-to-obtain Pokemon in Generation III, as its evolution requires max Beauty.
- knows Toxic, which is learned through a TM that can only be obtained once. Since Tentacruel also knows Toxic, you must either breed to pass the move down or trade the TM from another game.
- knows Ice Beam, which is learned through a TM that can only be obtained once.

# Cynthia #

## Diamond/Pearl ##

Spiritomb...

- is a difficult-to-obtain Pokemon in Generation IV, requiring interactions with another game in the Underground.
- knows Silver Wind and Embargo, which are both learned through TMs that can only be obtained once.

Roserade...

- knows Energy Ball, which is learned through a TM that is only available in post-game.
- knows Sludge Bomb and Shadow Ball, which are both learned through TMs that can only be obtained once (before post-game in the case of Sludge Bomb).
- knows Extrasensory, which is learned only as an Egg move.

Gastrodon...

- knows Earthquake and Stone Edge, which are both learned through TMs that can only be obtained once (before post-game in the case of Stone Edge).
- knows Sludge Bomb, which is learned through a TM that can only be obtained once. Since Roserade also knows Sludge Bomb, you must either breed to pass the move down or trade the TM from another game.

Lucario knows Earthquake, which is learned through a TM that can only be obtained once. Since Gastrodon also knows Earthquake, you must either breed to pass the move down or trade the TM from another game.

Milotic...

- is a difficult-to-obtain Pokemon in Generation IV, as its evolution requires max Beauty.
- knows Mirror Coat, which is learned only as an Egg move.

Garchomp...

- knows Earthquake, which is learned through a TM that can only be obtained once. Since Gastrodon and Lucario also know Earthquake, you must either breed to pass the move down or trade the TM from another game.
- knows Brick Break, which is learned through a TM that can only be obtained once before post-game.

## Platinum ##

Spiritomb...

- is a difficult-to-obtain Pokemon in Generation IV, requiring interactions with another game in the Underground.
- knows Silver Wind and Shadow Ball, which are both learned through TMs that can only be obtained once (before post-game in Shadow Ball's case).

Roserade...

- knows Energy Ball, which is learned through a TM that is only available in post-game.
- knows Sludge Bomb and Toxic, which are both learned through TMs that can only be obtained once (before post-game in the case of Sludge Bomb).
- knows Extrasensory, which is learned only as an Egg move.

Togekiss learns Water Pulse and Shock Wave, which are both learned through TMs that can only be obtained once.

Lucario...

- knows Stone Edge, which is learned through a TM that can only be obtained once before post-game.
- knows Shadow Ball, which is learned through a TM that can only be obtained once before post-game. Since Spiritomb also knows Shadow Ball, you must either breed to pass the move down or trade the TM from another game.

Milotic...

- is a difficult-to-obtain Pokemon in Generation IV, as its evolution method requires max Beauty.
- knows Mirror Coat, which is learned only as an Egg move.
- knows Dragon Pulse, which is learned through a TM that can only be obtained once before post-game.

Garchomp knows Earthquake, which is learned through a TM that can only be obtained once.

## Brilliant Diamond / Shining Pearl ##

Spiritomb is an annoying-to-obtain Pokemon in BDSP, requiring interactions with many unique NPCs in the Grand Underground.

All of Roserade's and Gastrodon's moves are learned via TM, which may be annoying.

Roserade holds an Expert Belt, which *may* be obtained once per day in Route 221.

Milotic...

- is a slightly annoying-to-obtain Pokemon in BDSP, as its evolution method requires max Beauty. However, this can easily be maxed out with Poffins made in Amity Square.
- knows Mirror Coat, which is only learnable as an Egg move.
- holds a Flame Orb, which is only available in post-game.

# Alder #

As Alder is a post-game exclusive fight in Black and White, there is very little that is gated away. The only issues here are the two trade evolutions, Accelgor and Escavalier.

# Iris #

Iris's Champion team in Black 2 and White 2 primarily uses Dragon-types and pseudolegendaries that are more difficult to level up.

Archeops is only available in post-game.

Aggron knows Double-Edge, which can only be learned if its evolution is delayed.

The two items used on her team are only obtainable with BP or Dream Radar.

# Diantha #

Gardevoirite is not available until post-game, and the TM for Shadow Ball is not accessible until post-game.

Additionally, you can only pick one of the fossil Pokemon, and must trade for the other.

# Professor Kukui #

Kukui uses a starter you did not pick, requiring a trade.

Pokemon Moon players will need to trade for Alolan Ninetales.

# Hau #

Tauros knows Iron Head, a BP tutor move.

# Trace #

Pidgeotite is post-game only.

In Let's Go! Eevee, Vileplume is unavailable.

# Leon #

Leon has a starter you did not pick, requiring a trade.

**If your chosen starter was Grookey:** Seismitoad knows Toxic, an Egg move.

# Geeta #

No difficulties here.


  [1]: https://pokemondb.net/pokebase/55876/pokemon-yellow-what-decides-what-gary-oaks-eevee-will-evolve