Adult Fear: Bucketloads of it. Imagine that such a nasty family lives in your neighbourhood. Or that your baby will be born with such horrible deformities. It's even present in-universe with Scully's uneasiness and anxiety over the issue.
Curb-Stomp Battle: The Sheriff does not go quietly when the Peacocks invade his house and fights back with a baseball bat, but unfortunately it may as well have been made of foam for all the good it does him.
Daylight Horror: Most of the episode takes place during the daytime and afternoon, including the climax. Though some of the freakiest moments, namely the inbred child's death and the Sheriff's murder, take place during the dark too.
Evil Cripple: A hissing limbless mother of the incestuous Peacock clan. She approves of her sons murderous behaviour. In her eyes they are only protecting their family.
Evil Matriarch: Mrs Peacock. She's involved in incestuous relationships with her sons. One of them is the father of the other two. Ugh.
Shout-Out: To The Andy Griffith Show. When Mulder expresses his desire for idyllic life in an American wholesome town, Scully says it would be like living in Mayberry. The local Sheriff's name is Andy Taylor (Mulder is kinda excited and asks if that's for real). His Deputy's name is Barney. But not Barney Fife (which Mulder hoped for, apparently). It's Barney Pastor.
Soundtrack Dissonance: "Wonderful, wonderful", a cheery tune by Johnny Mathis is used for a horrible murder and son/mother mutant sex, trying to conceive another child in a rocking car and all.