"There's four kinds of folks in the world. There's the ordinary kind, like us and the neighbours, there's the kind like the Cunningham's out in the woods, the kind like the Ewell's down at the dump, and the Negroes."

what rhetorical device or fallacy is this?

I don't know which rhetorical devices or fallacies you've studied, but this is an example of over-simplification.

The passage you provided is an example of a rhetorical device called stereotyping. Stereotyping involves making assumptions or generalizations about a group of people based on the characteristics of a few individuals. In this case, the speaker is categorizing people into four distinct groups based on their social status and race. It is important to note that stereotyping oversimplifies and dehumanizes individuals, often leading to unfair judgments and discrimination. To identify such rhetorical devices or fallacies, it is crucial to carefully analyze the language, context, and underlying message of the statement.