The Prime Mother was a title in Horta culture for the single Horta left to care for the Horta race's eggs at the renewal, a 50,000 yearly self induced extinction to clear the Horta's homeworld for the next generation of their species.
The Prime Mother was responsible for the care of all the eggs in the Vault of Tomorrow and effectively became mother to entire species once they hatched. (DS9 novel: Devil in the Sky; TOS episode: The Devil in the Dark)
In the 23rd century the Horta underwent a renewal which was interrupted when the Federation established a mining colony on the Horta's planet. The Prime Mother initially tolerated the Human presence, however in 2267 the miners broke into the Vault of Tomorrow and unwittingly destroyed thousands of eggs. In an effort to protect the eggs the Prime Mother attacked and killed fifty of the miners over three months resulting in the USS Enterprise being called in to investigate. A mind meld between the Vulcan Spock and the Prime Mother eventually resulted in an understanding of what the miners had done and the two races agreeing to work together. The miners came to know this Prime Mother as the mother Horta. (TOS episode: The Devil in the Dark)