Bat - Hang from the ceiling and drop down when you walk under them. The other kind will fly at you.
Spear Knight - Walks around, occasionally throwing spears in whichever direction it's facing.
Frankenstein - Walks around but starts running at you when you get close to one.
Panda - Jumps around. That's about it.
Bird - Flies around. Gets in your way.
Witch - Flies around. Drops eggs when you walk under one.
Turtle - Flies around. Gets in your way when you're on the rollercoaster.
Thunder Guy - Circles around the rollercoaster. Throws thunder at you.
Wind Guy - Same as above, except that it will blow wind at you.
Mine - Floats around slowly rotating. When you hit one, its spikes will fly either vertically or diagonally, depending on the direction they were facing.
Spark - Moves around platforms. You can't kill them.
Submarine - An annoying enemy that drops bombs on you when you walk under them.
Electric Eel - Swims around and makes a weird noise when they touch walls.
Ice Guy - Stands around and tosses popsicles at you when you get close.
Seal - Balances a bomb on its nose, which it tosses at you when you get close.
Snowman - Stands around and jumps toward you when you get close.
Frozen Duck - Runs at you and slides.
UFO - Flies around and drops Martians if you don't kill them quickly.
Martian - Walks around, usually following you. They can jump but not over holes.
Gorilla - Stands around and shakes the building and occasionally tossing stuff at you.
Spiderman - Yeah, really. Climbs up buildings and starts jumping around if you
don't kill them quickly.
Punk - Walks around on the subway cars, throwing their hair at you when you get close.
Masked Killer - I suppose this is a take on Jason or someone like that. They pop out of the grey boxes on the subway cars and then run around if you don't kill them quickly.
Cactus - Pops out of the ground and fires its needles at you if you take too long to kill it.