Because I'm sure people are dying to know my in-depth opinions on various Pokemon, here's what I personally consider some of the most fascinating creatures in the franchise:
GLOOM - grass/poison
This little guy was my single favorite pocket monster the second I laid eyes on him back in the late 90's, in an official Nintendo strategy guide. I'm a big fan of weird, rotten-smelling flowers (as you may surmise from my article on plant life), and Gloom has the added charm of constantly slobbering and having a cool, spooky name. I had an obsession with the word "gloom" as a kid and actually used it for several slimy fungus-like monsters, so this always felt like a pokémon I might have designed myself. It's unfortunate that his Rafflesia-based evolution loses the drool, and his other evolution is even cutesier.
PARASECT - bug/grass
While I still have a soft spot for Gloom, Parasect has slowly crept up into my #1 spot. He's the only parasite-themed pocket monster, and borrows from one of the most disturbing parasites in nature, the cordyceps fungus. Even more interesting is the fact that his unevolved form, Paras, is patterned after a cicada nymph, but never evolved into a winged, mature cicada-like creature. In the real world, a variety of cordyceps actually does kill cicadas in the nymph stage. What we're looking at here is the pokémon equivalent to a zombie. An entire insect species universally infected by a mind-controlling fungus!
SHEDINJA - bug/ghost
Based on a cicada but completely unrelated to Parasect, Shedinja is actually the molted exoskeleton left behind when Nincada evolves into Ninjask. Somehow, the exoskeleton comes to life as a ghost-type Pokemon, and has the unique quality of having only a single hit point, compensated with immunity to the majority of damage types in the game. If your opponent doesn't have any moves left that can touch him, they're completely screwed! I have to love the idea of a haunted insect exoskeleton.
MAGCARGO - fire/rock
I admire how pokémon would never just put a face on a pile of magma and call it a monster. Instead, they turn the traditional lava creature into a snail, with a shell formed as the lava cools off. It's genius, and it saddens me that he's actually one of the WORST fully-evolved monsters in the video games, due to a quadruple water weakness and low stats. I love you anyway, Magcargo, don't listen to those Smogon.com jerks.
Weezing - pure poison
Koffing and Weezing are such popular, iconic monsters that people don't seem to dwell on how peculiar they are anymore. The franchise is over a decade old now, but I still can't get over these guys. They're floating balloon-bomb-molecule heads filled with (and apparently CONGEALED FROM, if the Pokedex is to be believed) toxic gases, germs and dust. My favorite thing about Weezing is that third, faceless little pod fused with the two main heads. It's such a grotesque and disturbing mutant freak.
FORRETRESS - bug/steel
Though it looks like a bivalve, Forretress is an insect-based pogeyman; an evolution of the bagworm themed Pineco. It's also, apparently, metal-plated, and its claim to fame is its ability to explode. It's a caterpillar that wears a bomb. I used to wish it had a more bagworm-like appearance, but when they came out with another, completely unrelated bug/steel bagworm (sure as hell didn't see that coming) I still preferred Forretress.
SHUCKLE - bug/rock
Nobody can seem to agree on what the hell Shuckle is supposed to represent. His japanese name, Tsubotsubo, basically means "jar jar" and could be a reference to Fujitsubo, or barnacle. He certainly looks like a bunch of barnacles, but he lives in forests
, and feeds by allowing fruit to rot and ferment inside his shell. I always thought he might be a scale insect (a type of immobile, squishy bug which spends its whole adult life under an anchored shell) but I honestly don't know anymore. The English translations call it a mold pokemon. I guess it doesn't matter if he's a barnacle, a fungus or a scale insect, because those are all things I love anyway. His main theme seems to be the fermentation thing, as his shell produces various "potions" in the games. In Germany, they call him PottRott, which is awesome.
SWALOT - pure poison
I'm not sure whether I love Swalot's childishly simplistic design or wish he had just a smidgeon more detail, but either way he's certainly one of the more imaginative monsters for LACK of any distinct basis. He evolves from gulpin, officially referred to as a "stomach sac" pokemon. Swalot is a "poison sac" pokemon, so I guess these guys are giant body organs. His main gimmick is that his little mouth can stretch open to engulf almost anything, and rapidly digest it. Fairly horrifying, really. I particularly wonder why the artist decided his poisonous stomach-monster should be decorated like playing cards.
EXEGGUTOR - grass/psychic
I didn't think about it until recently, but Exeggutor is easily one of the weirdest mons of all time. A walking, psychic palm tree with goofy-looking heads for fruit, which apparently drop off and turn into Exeggcute, his unevolved form. Downright unnerving, really. It's like a triffid that can read minds.
SLOWBRO - water/psychic
I loved the recurring theme in the first 151 pokémon where various psychic types (once the most overpowered type in the game) were deceptively dopey and slow. I also love the idea of a goofy, sluggish sea-mammal with a grumpy seashell monster clinging to it forever. I'm pretty sure this is supposed to be a Sazaeoni. I used to think Slowking was less interesting because the shell turned into a hat, but when seen from behind, it actually still has the awesome grouchy monster face:
I still like Slowbro a lot better esthetically, but Slowking has some pretty awesome lore to it. Apparently the mollusc (a specially evolved Shellder) injects poison into its head which somehow makes it more intelligent.
HIPPOWDON - ground
I like when they mess with the status quo like this. It's a hippo monster, but it lives in sand and it hates water. It even BLEEDS sand, apparently. It also just looks totally bad-ass.
HYPNO - psychic
Easily EASILY the most frightening pocket monster. Forget all the legendary beasts and ghost types and firebreathing dragons; this is a pointy-nosed, squinty-eyed, hairy man-tapir that sneaks into houses at night to feed on people's dreams. It has an uncanny valley quality to it, and the same ugly-goofy feel that I apply to a lot of my own monsters. Part of hypno's lore also seems to be that he steals children, but I forget where this was started.