It's Vermilion. Both that and Fortree allow you to use fly, which isn't required for game completion.
Actually, since you can't earn an Earth Badge until you have all the other seven Kanto Gym badges, does that one really count? And the Fortree City example is only in the originals- in OR/AS, you're barred from progressing too far onto Route 120 until you've beaten Winona. And now for some more facts!
*Unown is the only Pokémon that is compatible with a TM that cannot actually have that TM used on it- it's only compatible with the Hidden Power TM, but the TM can't be used on it since it invariably already knows Hidden Power (and due to it being its only move, cannot forget it).
*OR/AS features areas with horde encounters that are ideal for EV training, since they only feature Pokémon that impart that particular EV; HP = Rusturf Tunnel (Whismur); Attack = Mt. Pyre's interior (Shuppet); Defense = Route 111 (Sandshrew, albeit not ideal due to the constant sandstorm and their Sand Veil Ability); Sp. Atk = Route 119 (Oddish); Sp. Def = Route 115 (Swablu); Speed = Route 104 (Zigzagoon, Wingull (south side)/Taillow (north side)).
*The only legitimate wild Graveler that cannot know Self-Destruct or Explosion are the ones findable via Rock Smash on Kalos's Route 13 or in hordes in Kalos's Victory Road. Any wild Graveler found in any circumstance other than these does know either Self-Destruct or Explosion (in other words, if you see a Shiny Graveler, you might wanna use a Master Ball).
*The Red Gyarados is the only in-game guaranteed Shiny that is not a Dragon-type, is in a region other than Unova, and must be encountered to progress the game's main storyline.
*Slowpoke has never been seen using an attack in the anime. It would share this distinction with Kakuna, except several of them in Azalea Gym used String Shot on Team Rocket.
*In the Japanese version of Generation II's games, Slowpoke's sprites featured a pattern on its belly like the one on Slowbro. This was removed in the international versions for reasons unknown.
*Goomy is said to be the weakest Dragon-type, when in fact, that distinction goes to Noibat, if we're talking in terms of base stats.
*Heatmor is the only Fire-type unable to learn Flame Charge (not counting the ones that aren't Fire-types by default, such as Rotom and Castform).
*Basculin is the only Water-type unable to learn Water Pulse (not counting Magikarp because lol Magikarp's movepool).
*The mysterious truck in Vermillion Harbor is in the remakes. It still serves no purpose, but there is a Lava Cookie not far from it- the only Lava Cookie in the game.
*Diamond and Pearl were not the first Pokémon games to add "A" to the "wild Pokémon appeared" message- that distinction goes to Pokémon XD.
*Pokémon that have obtained any Ribbons in the Orre region are considered special Pokémon and cannot be traded over the GTS (basically anything that used to be a Shadow Pokémon or was used for clearing the Mt. Battle challenge).
*Of all the Ice-types Generation V introduced, the only one with more than one type is also the only one that is a legendary.
*When a Shelmet and Karrablast are traded, if one of them holds an Everstone, neither one will evolve, regardless of whether the other one has an Everstone.
*If a Kadabra is traded while holding an Everstone, it will still evolve. For the record, this is not normal behavior for trade evolutions and Everstones. My opinion, this crap has Uri Geller's stink all over it. This has been a thing since at least Generation IV, possibly even earlier.
*FR/LG introduced rewards for catching 20, 40, and 60 Pokémon, whereas the originals merely had rewards for 10, 30, and 50 (20 got you an Everstone, 40 got you an Amulet Coin, and 60 got you the National Dex when you beat the game).
*Professor Oak will give you 5 Poké Balls in the original RBY, but in order to claim them, you must beat your rival on Route 22 first, and cannot have acquired any Poké Balls or obtained a Pokémon other than your starter. This event was so obscure that the testers overlooked a glaring error with the text associated with said event (one of the lines overlaps the previous one).
*In Black and White, all Basculin had Adaptability and Reckless as Abilities, regardless of form, when in fact Blue-Striped Basculin were supposed to have Rock Head instead of Reckless. The only Rock Head Basculin available in those games was from an in-game trade. This was rectified in B2/W2, and any Blue-Striped Basculin with Reckless gets their Ability changed to Rock Head once they're put through Bank.
*All legendaries in Kalos are Shiny-locked, as is the Lucario Korrina gives you, and the Snorlax on Route 7 for some strange reason.
*Escavalier gains the Steel-type due to swiping Shelmet's shell, but Shelmet is not a Steel-type despite having the same shell.
*The glitch Pokémon LM4 evolves into Clefairy at level 18, and the resulting Clefairy immediately evolves into Nidoking. Both evolutions are Pokémon whose regular evolution lines involve Moon Stones.
*In Generation I, viewing the stats of non-hybrid glitch Pokémon while they are in the PC will transform them into half-Rhydon hybrids, allowing them to learn all the TMs Rhydon can learn. Leaving them at the daycare changes them back. In this way, it is possible to, among other things, get a Missingno. to learn Hyper Beam.
*The Koffing and Gastly lines are unable to learn Sludge Wave via TM, yet were available with the move from the Dream World. Also, an event Gengar with Sludge Wave was given out.
*Deerling and Sawsbuck cannot change form in Generation VI. Without using Pokémon Bank, only Spring Deerling/Sawsbuck are available.
*Despite Kalos introducing the entire concept of hordes, Trevenant and Carbink are the only Kalos Pokémon to actually appear in horde encounters.
*In Kalos, horde battles cannot happen in extremely tall grass. In Hoenn, extremely tall grass is the only place horde battles
can happen without Sweet Scent or Honey to summon them.
*Weedle is the only Pokémon unable to learn any Normal moves. It arguably shares this distinction with Unown- while Hidden Power is classified as a Normal move, it is in practice anything
but a Normal move (or a Fairy move).
*Every odd-numbered generation of Pokémon has introduced a two-stage line of Grass-types based off of mushrooms that can have Effect Spore and learn Spore, while every even-numbered generation has introduced at least one new Eeveelution.
*Even-numbered generations are the only ones that have introduced new evolutions for old Pokémon. Generation I couldn't, because there
were no old Pokémon; Generation II gave us Crobat, Espeon, Scizor, etc.; Generation III gave us a couple of pre-evolutions, but that was it; Generation IV gave us Honchkrow, Mamoswine, Gallade, etc.; Generation V was all about that fresh start stuff and trolled us with Alomomola (poor Luvdisc); Generation VI gave us Sylveon.
*The Dream Ball actually has the same catch rate as the normal Poké Ball; the common belief about its catch rate stems from it only being usable in the Entree Forest, where
everything works like a Master Ball.
*Due to the way Poké Balls are passed down via breeding, it is impossible to get anything with its Hidden Ability in any Poké Ball introduced in Generation II.
*The Paras and Venonat lines hold the record for being able to legitimately be in the most varieties of Poké Ball- they can be caught in the Bug-Catching Contest, in the wild in G/S/C/HG/SS outside of said Contest, and in the Entree Forest in Generation V- only missing the Cherish Ball, due to having never been given out in a Nintendo event.
*Delibird has two different Abilities that do the
exact same thing. OK, one has a field effect and the other doesn't, but
who cares?
*The Gothita line can learn Thunderbolt, but not Thunder. The inverse holds true for the Solosis line.
*Deoxys is
not the first formerly event-exclusive legendary to lose said status- that honor goes to Mew, who is readily available (albeit a bit tricky to unlock) to anyone with a copy of My Pokémon Ranch.
*Voltorb and Electrode's Shiny coloration references the Great Ball. Foongus and Amoonguss's Shiny coloration references the Master Ball.
*The Murkrow and Drifloon lines are the only non-legendary Pokémon findable in the wild in OR/AS that cannot be chained via Dexnav, due to only being found while soaring in the sky.
*Until well into Generation V, the Glameow and Stunky lines were only available in Diamond and Pearl.
*There has been a different Pokémon Pinball title once per generation for the first three generations, each on a different platform- Generation I had Pokémon Pinball for the Game Boy Color; Generation II had Pokémon Pinball Mini for the Pokémon mini; Generation III had Pokémon Pinball: Ruby & Sapphire for the Game Boy Advance.
Didn't they also give it different forms for similar reasons, to make it seem like there were even more Water-types in the game?
<---- Yes, this is a backsprite. It looks almost identical to its fronsprite, but it is actually a backsprite.