The fact that Leviticus 18:23 bans bestiality suggests that human beings interbred with non humans enough for God to say to stop. Outside the Bible is a history of the Canaanites engaging in bestiality as well as other sexual immoralities on a regular basis. Egypt and Greece recorded bestiality in their art.
I don't recall anything specifically in history about interbreeding with neanderthals. But given the various history of mankind engaging in sex with different kinds of animals, neanderthal interbreeding wouldn't surprise me.
Since Neanderthals weren’t animals, it’s not surprising that it wasn’t mentioned.
Biologically speaking, Neanderthals were humans.
We’re all God’s children in the dark.
Neanderthals were not animals-they were human beings...