The term used to be “unhuman.” Now, it’s “inhuman.” I’m not sure exactly when it changed, certainly by the 1970’s. The etymology leads me to believe that in earlier decades some writers followed the trail from the old French word “humain” and so unhuman is correct. At a later time we accepted that human is from the Latin – “humanus” and so, inhuman is the right term. However, upon deeper investigation, I discovered that the older term, unhuman, is the correct one for science fiction monsters. “Humain” in old French means “of or belonging to man.” In Latin, “humanus” is both “of man, human,” and “humane, philanthropic, kind, gentle, polite; learned, refined, civilized.” So, if you act monstrously your behavior is inhuman. However, a lizard man is unhuman.
Thanks to Edmond Hamilton for the tip–off.
Reference — https://www.etymonline.com/word/Human