There was a time when the names of some of the original 151 Pokémon from Pokémon Red and Blue were still being decided, as many of the original Japanese Pokémon names wouldn't have worked. These names were changed to the ones that fans are familiar with, but they were close enough to being chosen that they were shown on early promotional material for the games. This material has since leaked online and there must be an Arceus looking out for Pokémon fans everywhere, because some terrible names were among the original choices, and they were eventually dropped.
The localizers of the Pokémon video games have a tough job, as they have to create names that match the theme of the Pokémon while still being memorable in the target language. There's also the issue of hitting the curse word filter on the Global Trade System, which prevented Bulbasaur from being traded in Pokémon Gen V. There are a lot of considerations that need to be made, and they have to be made for lots of different Pokémon. The Pokémon are the stars of the show and a bad name can diminish their appeal in the eyes of the fans. The games also have to be localized in a number of different languages, with their own teams trying to come up with names that work in their native tongue.
According to Bulbapedia, early promotional material for Pokémon Red and Blue made the rounds before the games were released. There were some beta Pokémon Red and Blue names that were awesome, such as Rattata being Rattatak, Paras being Parasyte, Gyarados being Skullkraken, and Vulpix being Foxfire. These names were a better fit than what the Pokémon were actually given in Pokémon Red and Blue, but ultimately it might be for the best that some of these early names were scrapped, as there were a few terrible choices among the line-up. It's possible the names were given a closer look before the anime was dubbed and they were changed as a result of sounding silly when said out loud.
The Growlithe Line Were Flamie & Blaze
A recurring theme among the bad beta names is their lack of imagination. That, or they simply don't gel with the theme of the Pokémon. In Pokémon Red and Blue, Growlithe's name mixes growl and lithe, as it's a dog-like Pokémon that is swift on the battlefield. Its evolution is Arcanine, which mixes arcane with canine, speaking to Arcanine's nature as a powerful dog-like Pokémon. The beta names for these Pokémon weren't as impressive. Growlithe was called Flamie (coincidentally, already the name of the furry flying dragon from SNES classic Secret of Mana, albeit without the extra M) and Arcanine was called Blaze. These are very general fire-themed words and could have been used for any number of different Pokémon. The name Blaze is especially non-descriptive, as it could also be used for Moltres or Magmar without needing to be tweaked.
The Omanyte Line Were Ess & Kargo
The Omanyte line has achieved a much improved status online, thanks to the Helix phenomenon in now-legendary Twitch Plays Pokémon. The line has since been revived from extinction, as wild Omanyte can be found in the Crown Tundra part of the Galar region. If the original pitched name for the Omanyte line had been used, then fans might not have taken to them as much. Omanyte was originally meant to be called Ess, and Omastar was going to be called Kargo. This was meant to be a reference to escargot, a French dish composed of cooked snails. It seems that the localizers thought that the Omanyte line were based on snails, instead of fossilized ammonites, which is where their new name came from. Ess would have been a particularly silly name for a Pokémon, and the other fossil duo from the same generation nearly suffered from a similar dumb naming theme. The escargot naming idea would later be used for Magcargo in Pokémon Gold and Silver, with is a snail composed of magma and rocks, and Escavalier in Pokémon Black and White, which is a knight-themed snail.
The Jigglypuff Line Were Pudding & Custard
Most of the Pokémon in Pokémon Red and Blue were given new names during the English localization process. Only a few Pokémon kept their original Japanese names, such as Pikachu and Mewtwo. One that almost had a name closer to its Japanese original was Jigglypuff. In Japan, Jigglypuff is called Purin, which refers to pudding (or especially custard). The beta name for Jigglypuff was going to be Pudding, with Wigglytuff being called Custard. This is one of those instances where the name was chosen without considering how it would have sounded in the anime, especially as Jigglypuff was a recurring character throughout the early seasons. Thankfully, this name was changed before the launch of Pokémon Red and Blue, otherwise, Ash Ketchum would have spent years complaining about Pudding.
The Koffing Line Were NY & LA
It's clear what the intention behind Koffing and Weezing's original names was meant to be. Koffing was originally going to be called Ny and Weezing was going to be called La. These names were references to New York and Los Angeles, and how they have high levels of air pollution. This fits with the Koffing line being all about poisonous gases. The real question is how these names would have been pronounced? Would Koffing have been called Ny (pronounced Nye) or N.Y. (pronounced Enn Why), and would Weezing would have been called La (pronounced Lah) or L.A. (pronounced Ell Aye?) There's also a chance that residents of New York and Los Angeles may have taken offense to villainous gas-spewing Pokémon being named after their home.
The Kabuto Line Were Att & Lantis
Omanyte and Omastar weren't the only fossil Pokémon that were almost landed with bad names in Pokémon Red and Blue. Kabuto and Kabutop were almost called Att and Lantis. This references the fabled lost city of Atlantis, which can tangentially be linked to Rock/Water-type fossil Pokémon. It's more Kabuto that would have suffered in this situation, as Att isn't a great name. Lantis on its own could be a cool name for a Bug-type fossil Pokémon in a future entry in the series, but it doesn't work as well for Kabutops. In the end, the line was given its original Japanese name, as kabuto is Japanese for helmet, which matches the shell design of its body.
Source: Bulbapedia