For everyone other than the three people who got this joke right away, one of whom may have sort of smiled at it, and especially for my parents, whose readership is always uppermost in my mind when I use the word horny in a comic, an explanation. See, back in the early days of Dungeons and Dragons, the creators made up a bunch of monsters the dungeonmaster could sic on you, the player. They published lots of books detailing the behavior of these creatures, the Monster Manual, the Fiend Folio, and so on. Each monster got stats and a black-and-white illustration, like this, and it was all super-dorky and low-budget and charming:
Click the gibbering mouther to go to the funny Something Awful review of early D&D monsters I kiped it from
Since the creators had to come up with so many of them, a lot of these monsters were kind of phoned in. Some of them, for example, were just regular old dinosaurs. Several were amorphous blobs or sheets or clouds that were easy to draw. And the creators especially liked unlikely hybrids of completely disparate animals, the best-loved of which is the owlbear (which I made an obscure joke about several years ago). The owlbear's entry informs us that its existence is "probably the result of genetic experimentation by some insane wizard." (Insane wizards were very busy in early D&D, creating hybrid monsters and subterranean labyrinths complete with magical booby traps for the hybrid monsters to live in.) So that's, you know, the joke. Insane wizard. It's a good explanation for anything, really.