In 2254, Captain Christopher Pike of the starship USS Enterprise had a violent encounter with a Kalar on Rigel VII, during which three of his own crew members, including his yeoman, were killed. Captain Pike later blamed himself for this, claiming that his own complacency was the reason the Kalar had been able to successfully attack them.
While en route to the Vega colony to hospitalize some of his crew who had been injured in the incident, the Enterprise was diverted to Talos IV, where Captain Pike was kidnapped by the Talosians. The Talosians used their illusion-creating powers to force Pike to relive the battle on Rigel VII, creating an illusory Kalar and this time adding the Human female Vina to the illusion, in an attempt to manipulate Pike into developing an emotional bond with her. (TOS: "The Cage")
The story outline for "The Cage" (as reprinted in The Making of Star Trek, pp. 47-65) includes an illusion similar to the one in which Pike is challenged with protecting Vina from this type of warrior. However, that early version of the scenario incorporates multiple such beings, described in the outline as "hairy manlike creatures," bipedal and armed with medieval weaponry that is not specifically described. (The Making of Star Trek, p. 50)
In the second revised final draft script of "The Cage", the Enterprise's commanding officer (at that point known as Captain James Winter) mentions the Kalar by saying its species name, during a discussion with Doctor Philip Boyce. However, the name "Kalar" is said only once in that episode's final version (in footage that was reused in "The Menagerie, Part II"), spoken by Vina, shortly before the reenacted battle scene. The conversation in which Pike talks about the Kalar with Dr. Boyce is still present in the final version of "The Cage" (as well as in "The Menagerie, Part I"), though he indirectly refers to it as "one of their warriors."
In the script for "The Cage", the sound produced by the Kalar is initially described as "a strange bellowing like a man-beast creature in a rage." When the Kalar first appears in the episode, it is physically characterized as "a huge Neanderthal-like figure [....] It is humanoid, huge, hairy, with carnivore's fangs, clad in a strangely shaped armor breastplate and helmet, brandishing a deadly looking mace."
The name of this species is spelled "Kaylar" in the official reference book Star Trek Encyclopedia and the closed captioning on the 2005 DVD set erroneously interprets Vina saying the name as "killer" rather than "Kalar." However, the name is spelled "Kalar" in the script of "The Cage". James Blish, in his adaptation of "The Menagerie, Part I" and "The Menagerie, Part II" as "The Menagerie" in the book Star Trek 4, also noted that the alien species was the "Kalar." Blish worked from the actual scripts used in filming, noting that they were "heavily revised in various handwritings (and in which Pike confusingly appears from time to time as 'Captain Spring' and 'Captain Winter')."
The reference books Star Trek Maps, The Worlds of the Federation and Star Trek: Star Charts, which all used the "Kalar" spelling, describe the Kalar as pre-industrial. Star Trek Maps (p. 29) and The Worlds of the Federation describe the species as rated D-plus on the Richter scale of culture, while Star Trek: Star Charts (p. 54) describes Pike's encounter as the first contact with the species, details the Kalar as being the dominant species of Rigel VII and characterizes the world as being quarantined without any aligned political system, pending development of warp drive. Whereas Star Trek: Star Charts (p. 54) gives Rigel VII a population of 725,000, Star Trek Maps (p. 29) cites a population of only 10,000 for the planet.
The Star Trek: Early Voyages comic "Our Dearest Blood" greatly expands on Pike's original encounter with the Kalar, and Kalar culture in general. In the comic, the Kalar are shown to be a warrior caste in a humanoid Rigelian species. As Rigel VII aims to join the Federation, the Kalar are threatened with being disbanded and conduct a coup. The attacks made by the Kalar in the coup are those which lead to the casualties and fatalities described in "The Cage".
The novel Burning Dreams expands on Vina's point of view of the Talosian's illusory Kalar encounter. It describes her playing up to her role as damsel-in-distress to assist in the Talosians' plan to have Pike fall in love with her and stay on Talos. From her viewpoint, she screams melodramatically and purposefully knocks things over to attract the attention of the Kalar, to try and draw Pike into the illusion by being too distracted with defending her to rationalize the Kalar as not being real.
The TNG comic Intelligence Gathering: "A Matter of Dates" depicts a group of Kalar co-founding a new Federation colony with the Rigelians in the 2360s. In the comic, the Kalar work as laborers, building and servicing the infrastructure of the colony. Unfortunately, the cooperative venture is soured when the Rigelians try to renegotiate their contract with the Kalar workers, to give the Kalar less compensation for their work. This act provokes the Kalar into repossessing the dam they had built, which provided power to the colony, which in turn leads Starfleet to send in the USS Enterprise-D to mediate the dispute.
Both comics and the novel used the spelling "Kaylar" for the name of the species.