Not entirely horror since it includes Conan and the guy with the triangle (i can't figure out who he is)(Update: It's Gullivar Jones, Warrior of Mars. See the comments here.).

I think the triangle player is Marvel Boy. Those look like the Quantum Bands on his wrists and I remember a more recent story explaining the whole three gold orbs thing on the front of his costume. He must have appeared in that (Unknown Worlds of) Science Fiction magazine.

Marvel Boy/Quasar was my first thought too. But the modern Wendell Vaughan version definitely didn't appear until Cap #217, which was some 3 years after this ad ran. And when he first appeared, he had the flame icon on his chest like the original 1950s version, which would also rule out this being the original version. The Marvel Chronology Project lists Unknown Worlds of Science Fiction on their non-canon list, and i went through the covers on GCD and the issue titles and listings on UHBMCC and it doesn't seem like they had any recurring characters or reprints from the Marvel Boy series. It sure looks like a modern Quasar, but i don't think it's him. Weird coincidence though.

Looks like Killraven to me...but more importantly, who knew that Alfredo Alcala could do such cool looking caricatures?

Very Jack Davis-y

Per Arndt this one is:
"Marvel Magazines Ad [Alfredo Alcala] 1p [Caricatures of Dracula, Conan, Werewolf By
Night, Frankenstein’s Monster, Gulliver Jones, Simon Garth, Satana & Moebius are
featured.]" So again Arndt's ID for the mystery man is Gullivar.