While the Pokemon anime does its best to ensure that many creatures are given the spotlight and can shine both in and out of battle, the RPG series is less capable of doing this simply due to how the game's mechanics and a Pokemon's stats work. This often results in many Pokemon being ignored entirely by the end of the mainline games that are held onto for dear life in the anime.
This normally happens with pre-evolved Pokemon that want to prove just how strong they can be without evolving, such as the many starter Pokemon Ash has had that didn't evolve, though there are many other Pokemon who shine in the anime but are complete letdowns in the video game series.
10 Eevee
Eevee is something of the secondary mascot of Pokemon, unofficially so in the early days of the RPG series simply due to how beloved it and its evolutions were to most players, but even more so now that it had a starring role in the Let's Go Eevee game. However, while this particular Eevee and just about every other Eevee in the anime are shown to be incredibly powerful, despite their small size, the reality is that Eevee isn't all that great in Pokemon, at least not anymore. There was a time when its signature Z-Move, Extreme Evoboost, was one of the more powerful Z-Moves out there and could do some serious damage when paired with any decent Pokemon and the move Baton Pass, but even then, it was a very niche strategy that not many players got on board with.
9 Farfetch'd
Even before Galarian Farfetch'd was introduced to the world, the original Farfetch'd was generally shown in the anime to be a powerhouse fighter capable of some seriously impressive feats with its leak. In the games, these displays couldn't be further from reality, as it is easily one of the worst Pokemon to come out of generation one. Galarian Farfetch'd could have rectified this thanks to getting an evolution, Sirfetch'd, but it was so underwhelming compared to just about every other Galarian Pokemon introduced in Sword & Shield that even a new form couldn't save it.
8 Mimey
In one of the earlier episodes of Pokemon Journeys, Ash makes a point of bringing his mother's Mr. Mime, nicknamed Mimey, to a Pokemon battling competition, much to the surprise of just about everyone present. Despite not being shown as much of a battler beforehand, Mimey is able to easily defeat an imposing Hariyama in seconds, surprising even the announcer with its power.
While Mr. Mime isn't bad and was, at one point, one of the better Psychic-type Pokemon in Red & Blue, its abilities weren't able to keep up with the growing roster of Psychic-type Pokemon over the years and it has faded into obscurity, even after gaining the addition of the Fairy-typing in X & Y.
7 Buizel
Dawn's Buizel, which was later traded to Ash due its love of battle, was a surprisingly powerful Pokemon all things considered. Sinnoh was a shining moment for Ash, with some of his best battles in the entire series occurring in this region, some of which involved this water weasel. While Ash's Buizel was able to pull off some impressive wins, its video game counterpart is far less impressive. Even after it evolves into Floatzel, it's a very underwhelming Water-type that is overshadowed by just about every other Water-type Pokemon in Sinnoh.
6 Gible
Like Ash's Buizel, his first-ever pseudo legendary Pokemon, Gible, was a surprisingly powerful Pokemon. Aside from being able to take a beating from a Dusknoir and a Shuckle that had used Power Trick to gain incredible strength in the Sinnoh League, Gible was able to come out on top thanks to its tenacity and absurdly powerful Draco Meteor attack. While fully evolved pseudo legendary Pokemon are generally very powerful, their base forms are far less impressive and are normally comparable to base form starter Pokemon in terms of stats.
5 Psyduck
Despite being a pretty weird Pokemon, both in design and personality, Misty's Psyduck is often shown to be one of her more powerful Pokemon, at least when it can activate its psychic powers. After Ash and his friends from Alola visit Misty, it is clear that her Psyduck is still a bit of a flake in battle, but it is also quite a bit more powerful and was shown to easily take on two Pokemon at once. Psyduck in the games, while having some useful moves, is fairly lackluster, though its evolution, Golduck, did see a resurgence in Sun & Moon where there were few other good Water-type Pokemon to take advantage of the Rain weather condition.
4 Onix
In general, Onix in the anime is shown to be a powerhouse monster of a Pokemon, capable of wrapping up its foes and crushing them into submission with ease, including in its debut episode with Brock and Ash's first battle in Pewter City. While Onix has some incredible defenses in the RPG series, that's about all it has going for it and is a Pokemon that most players should generally avoid.
Onix's evolution, Steelix, is only marginally better thanks to its even more impressive Defense stat and the addition of the Steel-typing making it resist even more attacks, though it lacks the same Special Defence and Attack stats that make its evolutionary line one of the least useful in the RPG series.
3 Luxray
Luxray is a Pokemon that most players want desperately to be good in the mainline series, which is probably why it is usually shown to be a very strong Pokemon in the anime. Its lack of move diversity in the RPG series heavily lets it down, despite having an amazing ability with Intimidate, though in the anime this doesn't seem to do much to stop it from being a powerhouse. Clemont's Luxray, in particular, required Ash to use his most powerful Pokemon at the time, his newly evolved Goodra, as well as some move interactions that would never work in the RPG series in order to win in their gym battle.
2 Charizard
Charizard in the anime is usually used as a particular trainer's signature and most powerful Pokemon in their team, as shown with Alain's Mega Charizard X and Leon's Gigantamax Charizard. While Charizard isn't bad in the games, it's nowhere near as good as it is shown in the anime, having fairly middling stats without the assistance of an additional form and has only made an impact in the RPG series when it is given a power boost.
1 Pikachu
While it is said in the anime that Ash's Pikachu is a one-of-a-kind monster that is far more powerful than he should be, it is also a misrepresentation of the electric mouse species as a whole. Pikachu has been able to take on threats that should have mopped the floor with him, including legendary Pokemon like Tobias' Latios and Kukui's Tapu Koko. In reality, Pikachu does have its uses in the RPG series, but it's nowhere near as powerful as the anime would like its audience to believe and isn't' worth the time investment the majority of the time.