The earliest report of loch monsters in Scotland comes from St Columba (521-597) who warded off a fierce river horse or kelpie in Loch Ness. Columba or Calum Cille (in Scots Gaelic) was an Irish abbot who brought Christianity to Scotland and he is commemorated in a stained glass window in St Cumin’s church on loch Morar.
The Loch Morar monster is thought to be a benign water bull or tarbh uisge, associated with fertility and part of Scottish Gaelic folklore. Morag was first sighted in 1887 and from then until 1981, 34 further sightings were recorded, including sixteen with multiple witnesses. John and Dora Metcalf saw it numerous times and reported it to a team of London University researchers in 1970, see this picture and report. For more details about Scottish water folklore, read the Masters thesis of Veronique Heijnsbroek (2015).