The sprite for Geodude in Pokémon Red and Blue stands out for me above all the others because it looks almost NOTHING like the promotional art. It looks like a human head with curly hair and arms attached, and the way the arms are bent it looks like an umpire telling someone they're out. Hit the road, jack. Scram.
In the original Red/Green version Geodude's sprite has kind of the same face but the eyes are larger and the arms are only raised, so it looks like a lil child trying to look tough. It's so CUTE. Still not obviously a rock.
Meanwhile Cloyster looks nearly the same as it has ever looked since, except that this was before they flipped the shell opening vertical and added a spike on top like ho ho ho the kids won't know what that is! Better hope so. (Check that: the Red/Green sprite had the vertical version and then they probably decided they were going too far.)
Red/Green Rapidash is curious. Gamefreak doesn't always get the sprites for that one right, but for a flaming racing unicorn they usually manage to play up the flaming and racing parts appropriately. The Red/Green sprite is the only one that plays up the unicorn part and makes the Rapidash look mysteriously majestic.
Galarian Rapidash looks like a regular Rapidash ran through an ice cream factory. It looks like what happens to dogs when they're bred for Best In Show at Westminster. What have they done to you, my magnificent steed.
Venomoth has only improved with age but it's also the bug pokemon that looks most like a real-world bug, which is to say it's kind of ugly up close.
As for the Ghastly line................
Sometimes I think that the idea of that line was to show a creature slowly condensing from gas into a solid form, because Haunter's original sprites appear to have solid particles coming off them.
But I've read some speculation that their design was originally supposed to invoke an extradimensional creature slowly busting its way through to our world. So with Ghastly in its first sprite, it's probably supposed to resemble a gas, and it does, but you wonder if that's not some...thing shoving its head through from its own world. And then Haunter kind of looks like it got farther forward, and then Gengar looks like it got all the way through. No idea if that's true, but I can see it. Gengar in Pokemon Red/Blue/Yellow kind of looks like it's...still not part of our world. Not completely. It almost looks like a shadow of the real thing, cast onto our reality. Except with eyes.
Meanwhile in Red and Green Gengar's Sprite is -- what is that. What the hell is that. Why is it grinning at me. Is that fur on its back or something? Why is it just standing there grinning at me? What the hell does it want?
I think the design of that one is supposed to be some kind of Oni because it looks like it has fur and hair, and it's in shadow like you're running into it within the deep dark woods. It's creepy in its own way. Not quite the same way as the later sprites. Bulbapedia says that the design might have been trying to evoke Shadow People, which is where you're in a sleep paralysis and the part of your brain that registers other people has a glitch, so you see a person-shaped hole in reality --
Imagine how much more terrifying it would be if that shadow opened its eyes and grinned.
But Gengar looks purple now. Not scary at all. Mega Gengar looks scary because they make its shape more intense and they lit it from below, but it's more like epic-boss-battle scary, not "what the hell is that" scary. That being said -- when I'm playing the game and my pokemon are below the mid-tier levels, seeing a Gengar on screen still scares the hell out of me, because those things are tough and mean.
The sprite for Hitmonchan in Red/Green is --
Ha ha ha ha ha.
HA HA HA HA HA HA HA HA.
TOGETIC IS WATCHING DODUO POOP.