Irving Wallace

First appearance: Stagefright: Aquarius (1987)

A strange dance theater production about a homicidal maniac wearing an owl mask is preyed upon by a homicidal maniac wearing an owl mask. Art imitates life? Slow moving, cheap Giallo/slasher flick from Italy (of course who else makes Giallo flicks) really picks up near the end when the killer finally gets around to offing the rest of the cast. Also the killer just looks really awesome with the owl head mask.
Surprisingly I found the movie in its entirety on youtube.

The Death Corps

First appearance: Shockwaves (1977)

Some folks end up on a tropical island that houses a horrible secret. Underwater Nazi Zombies. It doesn't end well for anybody.
Stars Peter Cushing as an Nazi scientist who created the creeps. You'd think people would know better than to hang around any place where Peter Cushing is, especially if he's got a German accent.

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.