Horror Monsters (or Horror Monster Series, to be more technically correct) represents probably the most important vintage monster-themed trading card set. Although the overall design of the cards is nothing to write home about, there is something about the simplicity of the cards, not to mention the great movie still photos, that captures the imaginations of card enthusiasts and monster fans alike. This was Nu-Cards' only monster-themed card set (unless one counts the Dinosaur Series), but it does bear some similarities with Rosan's Terror Monsters set, including the sharing of a couple of images.
The card fronts for the first series features a black and white still from a monster movie, surrounded by a green border with the title of the movie at the bottom. The card number sits within a white skull graphic at the corner. The backs have a text box in the center, which is surrounded by an elaborate border featuring several ghoulish characters and the series title at the top, all in red ink. The box is labeled 'Horrible Jokes' and does, indeed, contain a horrible joke.
The second series card fronts have the same basic format as the first, except that the border is orange and the card number sits within a plain white circle. The backs continue the 'Horrible Jokes' format, but with a different border (containing different monsters) in green ink. One big difference between the two sets is that some of the second series cards feature humorous captions rather than movie titles (usually spoofs of contemporary advertising slogans), and some feature artwork rather than photographs.
The test series, which presumably came first, resembles the regular second series in that it contains the same back matter (though in red ink), and the card number on the front is in a circle. The front border, however, is a dark blue, while the front caption seems to have been typed out on a piece of paper, cut out, and laid onto the original color stat, resulting in a very crude-looking item (again, similar to Rosan's Terror Monsters).
Many of the first series cards came in 2-card panels (see at right), and command a premium when purchased that way. Also, a paper poster showing all of the cards (first series only?) was available through mail order for sending in wrappers; it was identical to an uncut sheet of the cards (a few of which are known to exist).