I is for Insects
Let me get one thing out of the way: I love insects. I've taken a fair share of courses about insects, and currently work in an insect systematics lab. They've gotta be the coolest little organisms out there, as far as I'm concerned. Fact of the matter is though, that they creep people out a lot.
This alone makes them an excellent enemy to have around. Insects are freaky little things to most people, and when they see an insect on them it often results in a scream or a sudden brush of the clothing in order to get the damn thing off of you. If people don't like them all that much in Real Life (tm), then why the heck should they be fans of them in video games?
Insects are used in tons and tons of games as creepy enemies, or as swarming enemies that just irritate the crap out of you. Gigantic insects are also a frequent favourite of video game designers, such as the giant ants in the Fallout series. They're just so damn different from humans that they freak people out a lot...the way they skitter around on their multitude of legs, or screech through their piercing mandibles.
For some reason, insects appear to be a good spacefaring enemy as well...although I guess technically they would then be counted as aliens, but I'm gonna go ahead and mention Galaga's cast of foes. Insects that fly in patterns and swoop down to destroy or steal your ships are the main antagonists of Galaga, a game that I play in arcades whenever I get a chance. And don't forget the gigantic Mothula in The Legend of Zelda: A Link to the Past...a giant moth that you fight in a small room lined with moving Spike Sliders. Sounds a lot harder than it is, though.
Anyway, insects are different enough from humans that they can be creepy and also frequently small enough that they can be used as aggravating adversaries that you just can't ever seem to reach unless you have a plan. Their appearance and agility makes them a good enemy to have around in any game, and don't even forget about the fact that they can often cause some shitty status ailment in RPGs, such as the infamous Poison. If you're creating a game and need some form of enemy, you always have insects as a reliable fallback!
(P.S. Centipedes and millipedes are not insects, and neither are spiders. If you thought they were...then...well, just run away from me quickly. RUN YOU FOOL!)
J is for Jelly enemies
You know what these are. You're playing a video game, such as a fantasy or sci-fi action adventure game, and suddenly you come across an enemy that is, at most, an amorphous blob with a face or something like that. You have encountered a jelly enemy.
These cronies are just good for bashing around, in most cases. Nothing is quite as satisfying as sending a Zol flying across the room with a sword slash in the original Legend of Zelda, only to watch it split into two smaller versions of itself (Gels). Jellies are oftentimes one of the first types of enemies you run into, and perhaps the best game to use as an example is Dragon Warrior.
In nearly every DW game, you leave the first town and prepare to level up. After wandering around for a short amount of time, you get into a battle. It's a blue Hershey's Kiss with a clown face on it. That's a Slime, and you're gonna be seeing a lot of them. They're pretty much the Goomba of the Dragon Warrior/Quest franchise, and people love the hell out of them.
Most adventure games have some form of jelly enemy...Battle of Olympus, the Zelda series, the Metroid series, StarTropics I and II, the Final Fantasy series, Star Fox, Shadowgate...those're the ones off the top of my head. Give me more time and I can probably think of tons more...but right now it's late and I'm tired so how about you FUCK OFF instead!