The biggest taboo in the world is of course to cross a human with a chimpanzee. I have heard rumours that somewhere in south america or china people have attempted to make this hybrid in secrecy. Of course, these are just rumours, only suited for a pseudoscience forum. But I was browsing the internet a bit and found this article. Although in the end the example didn't really turn out to be a humanzee, it is quite an interesting story.
Humanzee
who is volunteering to mate with a chimp then?
Humanzee
who is volunteering to mate with a chimp then?