Other mistake: Before the wolf changes back into Evil Ed you can see the black cables pulling the fake wolf across the carpet.
Other mistake: When Jerry is in the nightclub starring at Amy you can see his reflection in one of the mirror columns of the dance floor. He is a vampire and shouldn't have a reflection. (01:02:51)
Other mistake: When Amy and Evil are at Peter Vincent's house to convince him to help Charlie, you see them looking into a horizontal mirror that's on the wall on the left side of the screen. If you actually look at the reflection, Evil is on Amy's right. However, he's on her left in reality.
Chosen answer: I would assume a "thrall" basically a ghoul that is attached to the vampire. A ghoul in old lore was created when a vampire gave blood to a living person, but did not take their blood first. They were then their daytime slaves, doing errands, finding victims for them, and took care of the vampire's body when the sun was out.
Billy was to Jerry... what Renfield was to Dracula. Slave, daytime watcher.