Aliens : K, L
Aggressive warrior race and leaders of a vast interstellar empire, first contact between Earth and the Klingons took place in 2151 in the infamous "Broken Bow Incident" in which a Klingon pilot crash-landed in an Oklahoma cornfield, pursued by Suliban operatives. Since then, Earth had uneasy relations with the Klingon Empire until finally war broke out in 2266. When the Klingons attempted to use the planet Organia as a military base, the Federation attempted to intervene, leading to the formation of the Organian Peace Treaty. Under the terms of the treaty, each side must prove that one or the other can develop planets most efficiently, and unfortunately for the Federation, although the Klingons can be brutal and savage as times, they are most efficient. Stricken with a mutated strain of genetic engineering in 2155, the Klingon genome was slightly altered to be more "augmented human" causing the forehead ridges to subvert, although, the affliction finally shed itself in 2271.
A God in Mayan and Aztec legends, Kulkukan visited Earth centuries ago to bring knowledge and peace to the primitive natives. Kulkukan left Earth, but promised to return one day. Sending a probe to Earth, the U.S.S. Enterprise pursued it back to its point of origin, where Kulkukan captured four crewmembers. Over the course of time, Kulkukan had captured many specimens from primitive species where they, although in captivity, lived a life of safety and contentment. Captain Kirk convinced Kulkukan that humans were not as primitive as they once were. Kulkukan agreed and allowed them to leave.
Episode Reference - "How Sharper Than a Serpent's Tooth?"
The Kzinti have a taste for human meat and during the four wars they fought with Earth, all of which they lost, many a Starfleet officer found themselves a tasty meal for a hungry Kzinti. Having lost all four wars, the Kzinti were forced to sign the Treaty of Syrius which forbade them possessing weapons, aside from those on police vessels. The Kzinti hoped that a stasis box from the Slaver Empire would contain a weapon that would allow them to successfully wage war against the Federation. A Kzinti's body contains vertical bracing in their ribs and more than one heart.
Episode Reference - "The Slaver Weapon"
A highly advanced, slug-like race inhabiting the planet Lactra VII. The Lactrans communicate telepathically and have no spoken language. They captured zoo specimens from all over the galaxy and put them on display on their world. Usually non-sentient life-forms, the Lactrans decided that the Starfleet officers they had captured had enormous potential and decided to let them go, asking them to return in the far future.
Episode Reference - "The Eye of the Beholder"
A predator residing in the Vulcan Forge, the Le-Matya's bite is poisonous and usually results in death. One attacked the Young Spock's pet sehla and mortally wounded it, but the adult Mr. Spock was able to subdue the creature with two Vulcan nerve pinches.
Episode Reference -