The mother Horta, born around 44773 BC, was the last of the Horta species of Janus VI until stardate 3196 in 2267. Every 50,000 years every Horta dies except for one, who watches the eggs until they begin 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 belived 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 Vulcan 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.