The mother Horta, born around 44773 BC, was the last of the Horta species of Janus VI until stardate 3196 in 2267. Once every 50,000 years the entire Horta population died off, except for one, who remained to protect the eggs until they began to hatch.
When the Federation established a mining colony on the planet in the 2210s, the mother Horta initially tolerated the presence of the miners. But when automated machinery broke onto level 23 in 2267, destroying thousands of Horta eggs in the Vault of Tomorrow, the mother began to fear for her children. She retaliated by destroying parts of the machinery with her powerfully corrosive acid, and when a maintenance engineer came to repair it, she killed him.
Three months later, by stardate 3196, she had killed over fifty people, including five guards (such as Schmitter) placed to stop her. Pergium production on the colony had been driven to a standstill. Colony administrative head Vanderberg summoned the USS Enterprise for assistance, as the colonists could not locate or harm what they believed to be a mere animal. Enterprise science officer Spock deduced that the reason for this was because they were combating a silicon-based lifeform, which has been previously thought mere fantasy. He was able to modify the hand phasers of Enterprise personnel to function against silicon lifeforms.
Spock and Captain James T. Kirk were able to harm the mother Horta after it killed a security officer, but it escaped through a freshly made tunnel. Shortly thereafter, Kirk encountered it once more, but wounded, the mother did not attack him. Spock was able to initiate a mind meld with her, and using the knowledge she gleaned from the meld, she used acid to write "NO KILL I" in the cave floor. Spock revealed that the creature was actually a sentient creature protecting her children.
The mother had stolen the main circulating pump from the colony's PXK pergium reactor, but in exchange for the return of the pump, Doctor Leonard McCoy was able to heal her phaser wound with thermal concrete. Once the situation was explained to the miners, they were suitably remorseful, but Spock negotiated an agreement between the Horta and the colonists via the mind meld. The colonists would leave the Horta eggs alone, and the Horta would use their abilities to assist in mining operations.
Spock reported that the mother Horta had an extremely logical mind, and though she found the humanoid form displeasing to behold, she thought she could get used to it. Her favorite aspect was the pointed ears; Spock could not bear to tell her that not everyone had them. (TOS: "The Devil in the Dark")
The mother Horta was played by Janos Prohaska. Much of the information about the Horta's life cycle comes from the words of the Horta mother.
Producer Robert Justman recalled how Prohaska, who had created the Horta costume himself, made the pitch for the role:
We made a "spec" deal with Janos. If he came up with a really great creature for a script Gene Coon was writing, we'd rent it and hire him to play the part. Janos was back within a week's time with his custom-designed creature. It was a large pancake-shaped glob of gook with a thickened raised center and fringe around its circumference. It sure didn't look like much. As Janos took the glob out of sight to put it on, Gene Coon raised an objection "Bob, why are wasting time with this?" Suddenly, the blob skittered around the corner, making straight for us. Then it stopped, curiously, backed away, and rotated in place. The blob gathered itself up, quivered, made a whimsical up-and-down movement, grunted, and skittered away again – leaving behind a large, round white "egg". Coon was dumbfounded. He watched the creature giving birth. And when the creature suddenly turned and scurried back to nuzzle its "child", Gene was sold. "Great!" he exclaimed, "It's perfect! Just what we need." Then he excitedly hastened back to his office to finish writing the script. Gene Coon's "The Devil in the Dark" became one of Star Trek's most famous episodes. And Janos Prohaska played his own creation, one of Star Trek's most famous creatures, the highly imaginative and custom-designed mother Horta." (Inside Star Trek: The Real Story, pp. 214-215)
What neither Justman nor Coon had realized at the time however, was that Prohaska had actually already created the creature previously for The Outer Limits, first appearing in the final episode, and that he had only slightly modified the rubber costume with veins and the "fringe" for its Star Trek appearance. (Star Trek: The Magazine Volume 3, Issue 9, p. 73) Titled "The Probe" (with Peter Mark Richman), the Outer Limits episode's storyline was about survivors of a plane crash in the Pacific waking up to find themselves (and their life raft) on the floor of an alien spacecraft sent to collect terrestrial lifeforms. In this episode, broadcast in January 1965, the future Mrs. Horta was performing yeoman service as a giant cold germ threatening the hapless Earth people. (The World of Star Trek, 3rd ed., p. 74)