The Loch Ness Monster (or Nessie) is a plesiosaur (really a family of plesiosaurs) living in Loch Ness in Scotland, Europe on Earth (TOS novel Deep Domain).
Roberta Lincoln mentioned the creature(s) in the 1980s (TOS novel The Eugenics Wars: The Rise and Fall of Khan Noonien Singh, Volume 1. James T. Kirk mentioned it in 2269 (TAS novelisation Bem). In the early 2270s, Montgomery Scott's one-time lover, Andrea Manning created a psychological manifestation of the creature to attack Spock with (TOS comic ...Like a Woman Scorned!).