In the universe of the Star Trek television shows and films, the Gorn are a race of large, highly intelligent humanoid reptiles. The Gorn have their own interstellar empire, the Gorn Hegemony, and have access to highly advanced technology at least on a par with the United Federation of Planets, including warp drive, transporters, and disruptor weaponry.
The Gorn are extremely popular among fans, and are often name-dropped in TV episodes, novelizations, and fan fiction, but to date Gorn have appeared on-screen on only two occasions - the original introduction of the Gorn in the Original Series episode "Arena," in which James Kirk is forced by an alien race (the Metrons) to battle an unnamed Gorn captain to the death on a specially prepared planet, and the Star Trek: Enterprise episode "In a Mirror, Darkly, Part II" in which a Gorn makes a brief appearance in a tale set in the Mirror Universe. A Gorn also appeared in the Star Trek: The Animated Series episode "The Time Trap."
The most probable reason why the Gorn haven't made many appearances on screen is that they look just a little bit too ridiculous. The limited special effects of 1960s television meant that the original Gorn looked very, very much like an actor in a rubber lizard suit. The designers went for more realism with the Gorn that appeared forty years later in the Enterprise episode, but the result was possibly even more ridiculous, as the Gorn looked for all the world like a talking velociraptor stolen from the warehouses of the Jurassic Park films.
Nevertheless, the Gorn seem to maintain a small but devoted following on the internet, which continuously cries out for their inclusion in future Star Trek offerings. So far, they've had very little luck.