The designs of the original 151 Pokémon are iconic, but some of them have changed over time. All of them look different compared to their Pokémon Red and Blue sprites by virtue of changing hardware, but some of the changes were deliberate decisions on the part of the creators of the franchise.
The Pokémon franchise was originally designed with a Japanese audience in mind, which is why there was so much censorship when the Pokémon series received an international release. The Pokémon video games have faced numerous changes since the days of Pokémon Red and Blue, as the creators took the time to refine the designs that had become so recognizable across the globe.
There are some Pokémon from the days of Pokémon Red and Blue that had their designs drastically changed over the course of the series. Some of these happened as early as Pokémon Gold and Silver, as the producers took the experience they gained from making the first generation of games and altered the existing Pokémon to make them more aesthetically pleasing. Other changes to Pokémon designs didn't happen until much later.
Pokémon Changes: Dratini's Belly & Dragonite's Fin
Dratini's belly in Pokémon Red, Blue, and Green was striped, in a similar manner to Dragonite's belly. This was changed in Pokémon Yellow and has remained smooth ever since. Dragonite's back had a long fin on it in Pokémon Red, Blue, and Green, which was also present on in its front sprite in Pokémon Red and Green. This was changed in Pokémon Gold and Silver.
Pokémon Changes: Haunter Lost A Finger & Mr. Mime Gained One
Haunter originally had three fingers and a thumb in Pokémon Red, Blue, and Green, but this was changed to three fingers in Pokémon Yellow and every game since. Mr. Mime originally had four fingers, which lasted all the way to Pokémon Emerald, when a fifth one was added. It's possible that these changes were made due to the connotations of having four fingers in Japanese society, as it's often connected to organized crime.
Pokémon Changes: The Jynx Controversy
The changes to Jynx's design might be the most well-known outside of the Pokémon fanbase. Jynx's design was criticized for being similar to blackface imagery, which is why its skin was changed to purple after the first generation of games. Jynx's design was also changed retroactively, as its sprites were edited in the Virtual Console versions of the Gen 1 Pokémon games.
Pokémon Changes: The Chubby Pikachu
Pikachu's design was different in Pokémon Red, Blue, and Green, as it had a circle pattern on its belly. The original Pikachu was also chubbier, which is something retained in Pokémon Yellow, even though that game dumped the circle pattern.
Pikachu's design gradually slimmed down even more over the years. It's believed that the Gigantamax Pikachu design was inspired by its retro look.
Pokémon Changes: Golbat's Tongue & Poliwhirl's Gloves
These changes have been inconsistent throughout different Pokémon games. Golbat had a large tongue in Pokémon Red and Blue, but this aspect of its design kept changing. As of the past few generations of Pokémon games, the tongue is gone. Poliwhirl's gloves have also switched between finger gloves and mittens depending on the game. It has fingered gloves in Pokémon Red/Blue/Green, but switched to mittens in Pokémon Yellow. It has kept the mittens in recent Pokémon games.
Pokémon Changes: Gen 1 Sprite Oddities
A couple of Pokémon had bizarre design choices that were clearly wrong and didn't match the anime or official artwork. Machoke's head fins used to run all the way down its back in its Gen 1 back sprites, and these were removed in Gen 2. Gastly was originally a cloud of gas and it didn't gain its familiar ball design until Pokémon Yellow. Lickitung had a forked tongue in its Pokémon Red and Green sprites, but this was fixed for every other version of the games. Koffing's skull and crossbones were on its forehead in Pokémon Red and Blue, even though it was in the correct position (beneath its mouth) in Pokémon Green. Vaporeon also had several changes in all of its Gen 1 sprites, including black skin behind its ears and small fins on each of its legs, which were corrected in Pokémon Gold and Silver.