Canon Facts Backstage Info  Speculation

Every 50,000 years or so virtually the entire Horta species dies out, leaving a large number of eggs in a chamber known as the Vault of Tomorrow. This cache is guarded by a single individual until they hatch. On Janus VI the Vault was penetrated by a Federation mining team who destroyed many of the eggs, not realizing what they were. The Horta responded by killing many of the miners, leading to the dispatch of the USS Enterprise to assist with the situation.

Captain Kirk and Commander Spock were able to wound the Horta with phaser fire, and the creature was subsequently cornered by Captain Kirk. The Horta was able to communicate a plea for mercy to Kirk. Spock mind melded with the creature and was able to determine the cause of the conflict. The Enterprise's Chief Medical Officer, Doctor McCoy was able to repair the Hortas wounds by applying some cement to them. Kirk convinced the miners to stop destroying the eggs, and in return the Horta stopped its killing spree. The arrangement proved to be highly beneficial to both sides - once the Horta eggs hatched the first thing the young did was begin to tunnel through the surrounding rock, opening up huge mineral resources to access by the miners.