Zeltok was a class M planet located in the the galaxy's Alpha or Beta Quadrant. It was the homeworld for three extinct intelligent species, the humanoid Zeltok, a race of gigantic Zeltok ants, and a race of sentient Zeltok gorillas.
In 1465, an internal explosion destroyed Zeltok.
In 2265, several hours after passing through the Perseant Alpha system, the USS Enterprise encountered a temporal disruption that sent the Federation starship to the star system containing Zeltok, two weeks prior to the planet's recorded destruction.
James T. Kirk, Spock, Sankey and Joe Wells explored the surface aboard the Galileo shuttlecraft, discovering Karn, the last surviving Zeltok, as well as sentient races of ants and gorillas. Karn refused to be rescued, and a subsequent astral storm returned the Enterprise to the 23rd century, erasing the crew's memories of the incident. However, Wells' phaser was still missing, as Karn had taken it, leaving the crew with a minor mystery. (TOS comic: "Vibrations in Time")