There have been many changes to game mechanics in Pokémon since the original Red and Blue titles, from adding new battle modes and types to balance gameplay to giving certain Pokémon stat buffs or new abilities, giving them a fighting chance against the ever growing roster of creatures.
As Kanto was the first generation of Pokémon games, the Pokémon found in that region have had the most time to grow and become stronger with each new addition to the series. That being said, some of Kanto's natives weren't so lucky and ended up losing power rather than gaining it.
10 Stronger: Flareon
For a very long time, Flareon was seen as the oddball of the original Eevee evolutions. While it had the highest attack stat among the three, with it being a Fire-type meant that it could never take full advantage of those moves, as Fire-type attacks were considered special attacks until generation four. Diamond and Pearl teased a new lease on Flareon's life with the change of how attacks worked in conjunction with the attack and special attack stat, but it wasn't until X and Y that it was able to learn Flare Blitz, the best Fire-type move Flareon could learn for its stat distribution.
9 Weaker: Gengar
Gengar's status as a great Pokémon has never been contested, though its original appearance in Red and Blue left little to be desired, as there were no good Poison or Ghost-type moves for it to use. While it has grown significantly in power over the years with the addition of Shadow Ball and Sludge Bomb for great attacks, as well as gaining the Levitate ability in Ruby and Sapphire, it has recently been knocked down a peg. With the release of Sun and Moon, Gengar returned once again to the Pokémon series, though this time it didn't bring Levitate with it, instead being replaced with the far less useful Cursed Body.
8 Stronger: Snorlax
Like Gengar, Snorlax has gone up and down in power over the years as mechanics and movepools change. During its debut in Red and Blue, Snorlax was considered a terrifying special attacking threat with its access to Amnesia and a very diverse movepool common among Normal-type Pokémon.
While it has since lost this threat with the special stat split in Gold and Silver, it has gained Belly Drum in its place, a high risk high reward move that greatly benefits from its Hidden Ability, Gluttony. Snorlax was also one of the few lucky Pokémon to gain a Gigantamax form in Sword and Shield which gave it an even greater boost in power.
7 Weaker: Fearow
In the original Kanto region games, Fearow is the uncontested victor between itself and the other early game bird Pokémon, Pidgeot. Though neither of them hold a candle to Dodrio or the Legendary Birds, Fearow at least had one up on the arguably more popular bird. This would quickly change with FireRed and LeafGreen, as Charizard was the far superior Flying-type after its movepool had been updated, and Fearow would be knocked down a peg once again in Omega Ruby and Alpha Sapphire with Pidgeot gaining a Mega Evolution. As it stands now, Fearow is easily one of the weakest early bird Pokémon in the series due to how many Pokémon have changed around it.
6 Stronger: Butterfree
There was a time in the series when it was considered absurd to think that Butterfree was ever good for anything except taking down Brock's Onix in Red and Blue, since neither of his Rock-type Pokémon actually knew Rock-type moves. While it quickly became more and more irrelevant over the years, it has gained a surge of popularity and usefulness with the including of Gigantamax forms, with one of them going to Butterfree. With this new form, Butterfree went from one of the worst Bug-type options on a team to one of the most annoying dedicated Gigantamax options, as its signature move, G-Max Befuddle, was able to inflict either Poison, Paralyze or Sleep status effects on both opponents in a double battle.
5 Weaker: Alakazam
In generation one, Alakazam was the uncontested best Pokémon in the game, with its great speed and special stats making it very hard to take down without the aid of something like Snorlax, and even then it wasn't a guaranteed counter. While Alakazam has maintained its position as one of the best Psychic-type Pokémon in the series, it has also had to contend with Gengar's own improvements as well as newer additions to the series. In X and Y, Alakazam was gifted a Mega Evolution, which increased its already great attack and speed stats to absurd levels, but was unfortunate enough to also be in the same game as Mega Kangaskhan, which consistently used Sucker Punch in its moveset.
4 Stronger: Gyarados
Gyarados has never been a particularly weak Pokémon, though there was a during Gold and Silver and Ruby and Sapphire where it couldn't make full use of its Water-type moves. That handicap never really stopped it from being great, particularly with the addition of its ability in generation three, Intimidate.
Since then, Gyarados has only gained strength, with the inclusion of moves like Stone Edge, Power Whip, and Iron Head to its movepool making it a strong threat on any team. This power increased even further in Sword and Shield with the Dynamax mechanic, which not only catapulted Gyarados to the top of most player's teams, but has recently seen it banned from VGC tournaments alongside a handful of other Pokémon.
3 Weaker: Moltres
The Legendary Birds, though popular, haven't always maintained a level of power than normally comes with being a legendary Pokémon, with the exception of Zapdos. While Moltres was never really bad, it took a significant blow alongside Charizard with the introduction of Stealth Rocks, a move that would take of 50% of its health when simply brought onto the battle field. While Charizard has been able to keep its status as a good Pokémon, largely due to its numerous different forms, Moltres has been left behind and is no longer a good Fire/Flying-type option.
2 Stronger: Pikachu
Using a Pikachu in generation one was more of a novelty than a good team choice, as nearly every other Electric-type in the game was better in every way. Generation three hoped to change that with the inclusion of the Light Ball, but even then it was better to simply evolve it into a Raichu, which still sees some competitive play every now and then. Like many other Pokémon, Pikachu gained some power with the inclusion of a Gigantamax form, who's signature move, G-Max Volt Crash, guarantees paralyzing both opponents in a double battle, making it a viable dedicated Gigantamax option on many teams.
1 Weaker: Articuno
Like Moltres, Articuno suffered greatly from the inclusion of Stealth Rocks to the game, as it also takes 50% of its HP when it enters battle. While Articuno has also gained some valuable new moves with the likes of Freeze Dry, Hurricane, and Tailwind, its unfortunate type combination means it is likely to only get weaker as time goes on, though its Galarian form, which is Psychic/Flying-type, may give it a new lease on life that it desperately needs.