Yeah, I wasn't really talking about the archetypes of horror. Those already inform a lot of the Monster Manual anyway, but I'd agree that taking one of those and trying to make it a centerpiece isn't that interesting. I was talking abut the common elements of horror fiction - (paraphrasing the interwebs here) it explores 'malevolent' or 'wicked' characters, deeds or phenomena. It arouses feelings of fear, shock or disgust as well as the sense of the uncanny – things are not what they seem. There is a heightened sense of the unknown and/or mysterious. These elements are present in a lot of games, I just lean into them a little harder than some people. I also tend to include some elements of body horror - gross parasites, strange conditions, pernicious poisons. Those, used somewhat sparingly and often in conjunction with expanded exhaustion rules, tend to puncture some PCs sense of invulnerability without killing them.
Fair enough. Yeah, the archetypes inform a lot of what people call "monsters," but as you say the archetypes themselves tend to underwhelm in play--if only because the players recognize them as horror-monsters. I think there's a space where there are elements of horror but the setting is not unrelentingly depressing. I suspect we both aim for it, but our approaches (and possibly reasons) are probably different-ish. For every arc where the PCs are dealing with contagious derangements that can reshape bodies as well as minds (in at least one instance literally pulling the skin off a corpse to walk around) I try to arrange it so there's at least one where they're doing ... more conventional D&D things. If nothing else, the "conventional D&D things" are closer to what the players expected when they joined up.