Here is a visually festive monster which I do not think has ever made a second appearance -- the mighty Leviathan. Leviathan was created by the Plantman and unleashed on his native London in the second and third issues of the vintage Sub-Mariner run by Roy Thomas and John Buscema.
The story started in the second issue of the comic (my very first) when Namor quite logically meets up with and locks horns with Triton of the Inhumans. This was the first meeting of these two undersea characters,but of course hardly the last.
The pair battled each other and then the Plantman who is presented as a quite serious villain in this outing and not an absurdity as is the case sometimes. He has a scheme to use his herbivorous talents to rise up mildly sentient seaweed into a monster and unleash it on an unsuspecting London town.
The Leviathan is a proper monster and quite visually arresting, almost like a walking abstract. But of course it is defeated, as it must be and the Plantman loses and he ought. I don't think Leviathan has ever made another appearance. But once upon a time the Leviathan rampaged indeed.