So after posting in and looking at posts from this thread about mons that were more prominent in generations beside their own, this inspired a potentially new interesting to topic: the opposite of that, Pokémon who haven't been in any sort of prominence or relevance in any game or generation outside of their debuting generation. There are probably a few examples of these, aside from the starters who aren't meant to be relevant outside of their own games, but this is a potentially interesting topic to discuss.
Let me start off with two examples thus far:
Glameow and Purugly are a prime example of this imo. They debuted in Diamond and Pearl, but they were exclusive to Pearl. In spite of this, they had prominence as being the ace of one of Team Galactic's commanders (Mars) and being a regular sight from team Galactic, and being the version exclusive counterparts to Stunky and Skuntank. They were absent from both Platinum and HGSS outside of trading, but they were especially not prominent in any generation since. They haven't been in any regional dex since Gen 4, and they only show up in Mirage Spots in ORAS, and while Skuntank got to show up in XY and SwSh, poor Glameow hasn't, and is still Dexited in Gen 8 as of now (though it will obviously be back in BDSP).
Another prime example is Blitzle and Zebstrika. Blitzle is very much a prominent Pokémon in its debut game of Black and White, being an early game Electric-type that shows up early and becomes an incredibly cool zebra in Zebstrika, as well as being the ace of the well known Elesa. It's still prominent in BW2 due to being Elesa's ace, but it's also less prominent in that you can only find it in the wild readily evolved as Zebstrika: unlike BW1, you cannot raise it from a humble little Blitzle and you will need to breed if you want to use Blitzle, which is only possible in the post-game of BW2. Zebstrika hasn't appeared in any regional dex ever since, and is completely unavailable in Gen 7 without transfer and is also currently Dexited from Gen 8 (unless Legends brings it back, which I'm low key hoping it does, Zebstrika is so cool :( ). In Gen 6, much like in BW2, it is only available in the Friend Safari in XY's post game and the Mirage Spots in ORAS: in both cases, it's readily evolved as a Zebstrika, which means it's also less special to find since you don't get the luxury of evolving it from little Blitzle. This is one of those Pokémon who hasn't been prominent since its debut and has since been largely an afterthought, not showing up prominently in any games since (Excadrill and Galvantula come close to being as non-prominent, but had the luxury of finally returning in Sword and Shield's Galar dex, something Zebstrika hasn't had the luxury of having just yet).
Are there any other examples of this you can think of? Feel free to discuss!
Let me start off with two examples thus far:


Glameow and Purugly are a prime example of this imo. They debuted in Diamond and Pearl, but they were exclusive to Pearl. In spite of this, they had prominence as being the ace of one of Team Galactic's commanders (Mars) and being a regular sight from team Galactic, and being the version exclusive counterparts to Stunky and Skuntank. They were absent from both Platinum and HGSS outside of trading, but they were especially not prominent in any generation since. They haven't been in any regional dex since Gen 4, and they only show up in Mirage Spots in ORAS, and while Skuntank got to show up in XY and SwSh, poor Glameow hasn't, and is still Dexited in Gen 8 as of now (though it will obviously be back in BDSP).


Another prime example is Blitzle and Zebstrika. Blitzle is very much a prominent Pokémon in its debut game of Black and White, being an early game Electric-type that shows up early and becomes an incredibly cool zebra in Zebstrika, as well as being the ace of the well known Elesa. It's still prominent in BW2 due to being Elesa's ace, but it's also less prominent in that you can only find it in the wild readily evolved as Zebstrika: unlike BW1, you cannot raise it from a humble little Blitzle and you will need to breed if you want to use Blitzle, which is only possible in the post-game of BW2. Zebstrika hasn't appeared in any regional dex ever since, and is completely unavailable in Gen 7 without transfer and is also currently Dexited from Gen 8 (unless Legends brings it back, which I'm low key hoping it does, Zebstrika is so cool :( ). In Gen 6, much like in BW2, it is only available in the Friend Safari in XY's post game and the Mirage Spots in ORAS: in both cases, it's readily evolved as a Zebstrika, which means it's also less special to find since you don't get the luxury of evolving it from little Blitzle. This is one of those Pokémon who hasn't been prominent since its debut and has since been largely an afterthought, not showing up prominently in any games since (Excadrill and Galvantula come close to being as non-prominent, but had the luxury of finally returning in Sword and Shield's Galar dex, something Zebstrika hasn't had the luxury of having just yet).
Are there any other examples of this you can think of? Feel free to discuss!