Gorn
The Gorn are a reptillian species appearing in the TOS episode "Arena". They are essentially bipedal humanoids. They are much stronger than humans but not as agile or quick on their feet; they are apparently incapable of running.
Appearances
A Gorn starship attacks a Federation outpost that was supposedly set up in their territory. They lure Kirk, Spock, McCoy and the Enterprise's tactical officers to the surface of the destroyed colony where they ambush the landing party and their starship attacks the Enterprise with the majority of its combat personnel down on the surface.
Kirk is able to counter-attack the Gorn ground party with photon grenades found in the colony's armory. The Gorn starship beams up their wounded and retreats. Kirk and company return to the Enterprise and give chase, Kirk enraged and demanding they pay for their attack on the colony.
After some time, both ships are held helpless by the Metrons, another one-episode, allegedly non-violent super race who attempt to "solve" the problem by having Kirk and the Gorn captain resolve their issues in one-on-one combat on an unknown planet. However, to test their mental abilities, the Metrons place the raw materials for gunpowder on the planet. Kirk assembles them into a primitive cannon and cripples the Gorn captain but refuses to kill him. This impresses the Metrons, who spare both ships and return them to their own territory. Kirk realizes the Gorn may have actually been in the right, merely feeling they were defending themselves from invaders, and decides this disagreement would be a matter better left in the hands of the diplomats[1].
This is the only time the Gorn appear in the main Star Trek universe, although one did appear in the mirror universe in Star Trek: Enterprise[2], in spite of the race appearing to be completely unknown to the Federation in Kirk's time.
Threat assessment
The Gorn ship was able to withstand a full barrage of phasers and photon torpedos from the Constitution-class Enterprise indicating defenses comparable to those of the Federation. The Enterprise needed to push its warp drive to its limits to overtake the ship during the chase.
Their infantry seems superior to that of the Federation (not exactly saying much), as they were able attack the Federation landing party from a distance of approximately 1200 yards. They were able to detect Spock's tricorder and use the signal to bracket the Federation position with explosive fire from a similar distance.
However, the lack of any appearance in TNG-era Star Trek indicates that their territory is either very minor or they were destroyed some time after the events of "Arena".