The Davies' Baby

First appearance: It's Alive (1974)

The heartwarming tale of a hideously deformed, murderous baby and the family who loves him.

The Bag

First appearance: My Stepmother is an Alien (1988)

An alien comes to earth to figure out why earth attacked her home planet. She ends up falling in love with and marrying the scientist who did the attacking after figuring out it was an accident. The scientist's daughter figures out there's something wrong with her new mom and the weird eyeball thing that lives in her purse. Much fish out of water hilarity ensues. Originally pitched as a dark allegory
about child abuse.


  OK guys I'll get back to actual monsters from horror flicks soon. I promise

Zeus

First appearance: No Holds Barred (1989)

The story of an Evil TV network trying to capitalize on the popularity of WWF Champion of the world Rip (Hulk Hogan) by starting their own fight show where hideous human monsters beat the snot out of each other.  They find their star in Zeus. Zeus hates Rip. They eventually fight.
Weird trvia: Zeus actually ended up in the non-movie universe WWF.

David

First appearance: The Lost Boys (1987)

A single mom and her two sons move to the murder capital of the world. Vampiric hijinks ensue.
Features the greatest vampire hunters in movie history. The Frog Brothers, Edgar and Allen.

Molasar

First appearance: The Keep (1983)

Nazi's guarding an ancient keep awaken something horrible when trying to steal silver crosses embedded in the walls.  A mystical weirdo swings by to help a jewish academic and his daughter who are being held prisoner by the Nazis and forced to study the creepy keep. The weirdo's actually there to help the horrible thing. Weirdo falls in love with the academic monster. Horrible thing kills a bunch of Nazis proving that even ancient evil hates Nazis
Based on a pretty good novel by F. Paul Wilson. 

The Boogens

First Appearance: The Boogens (1981)

A bunch of weird octopus turtle things are released from an old silver mine 100 years after the army closed the mine due to a mysterious massacre. The boogens proceed to kill people. Stephen King gave the movie a good review in Twilight Zone Magazine, in case things like that are important to you.

Alien, hunter

First appearance: Without Warning (1980)

Have you ever heard of that movie 'Predator'? You know the one where an alien hunter stalks and kills a bunch of guys in the jungle? Yeah? Well this is kind of like that. Only its in the woods near a lake and the people fighting the alien are a crazy guy who may or may not own a gas station (played by Jack Palance, the guy not the gas station), a crazy army vet (Martin Landau) and a pair of useless teens. Instead of invisibility and high tech weapons, this alien throws weird, fleshy, frisbee parasites at people.  The alien's not it it very much.
Another interesting parallel to Predator, both aliens were played by TV's Misfits of Science's Kevin Peter Hall.

 

Father Harry Powell

First appearence: The Night of the Hunter (1955)

Can't believe it took me so long to get around to seeing this movie.
 It's about a homicidal maniac who believes god is telling him to murder widows and bimbos. He ends up being thrown in jail as a car thief, his cellmate is going to be hanged for killing a couple of people during a bank robbery. One night the man who will be hanged talks in his sleep revealing that the money he stole is hidden somewhere and his children know where. After the ersatz preacher is let out of jail he head straight to the bank robber's home town and seduces his wife. Terror and suspense ensue.