Green’s Dictionary of Slang

(US) a general term of abuse, lit. one who has sex with dogs.

[1975		C. Skinner Carol's Curious Passion (1988) 16: If people knew I was not only a dog fucker, but had sex with my own brother, they wouldn't respect me much].
1992		J. Mowry Way Past Cool 22: Them KKK dogfuckers likd to get us fightin so’s they could watch an laugh over it.