The Loch Ness monster was a mythical aquatic animal that supposedly lived in the murky Loch Ness in Scotland on Earth.
While searching for the missing contact party on Delta Theta III, Uhura reported that the anomaly they had been observing had completely covered the entire northern continent and that their sensors were unable to penetrate the region to locate the party. Scotty, who was in command, analogized the sensor deficiency by stating that "the Loch Ness Monster could get through that", due to the creatures' supposed ability to navigate the murky loch. (TAS: "Bem")
The myth of the Loch Ness monster was known on Earth in an alternate timeline in which the Nazis had conquered the United States of America. After learning that Captain Jonathan Archer was a starship commander from the future and that the Nazis had received help from aliens originating even farther in the future, Carmine sarcastically asked Archer if their next mission was to hunt down the Loch Ness monster. (ENT: "Storm Front, Part II")