The microscopic descendants of a lost Earth colony, shrunk over generations by the radiation of their unstable world and rescued by the Enterprise in The Terratin Incident.
The Terratins are the descendants of human settlers from a lost colony expedition, whose settlement was originally called Terra Ten. Strange wave emissions on their volcanic planet contracted organic matter, and over generations the colonists shrank until they stood a fraction of an inch tall, a change that became hereditary.
In the animated episode The Terratin Incident, the Terratins bombard the Enterprise with the same radiation to draw attention to their plight, as their world is tearing itself apart. Kirk and his crew begin to shrink until they can barely work the controls.
Kirk discovers that the transporter restores his people to full size, though it cannot help the Terratins. The Enterprise beams up the whole miniature city and carries it to a safe new world.