Such stories follow common themes though there would also be local variations. However, by and large, such stories were confined to the Highlands of Scotland where the preponderance of lochs would lie.
Three beasts dominate our thread of posts and that is the Water Horse, Water Bull and the Kelpie. The three are similar but sufficiently different to merit their own stories (though this did not stop writers from confusing them). But their major differences would be that the Kelpie inhabited rivers whilst the Water Horse was seen as a more aggressive beast than the Water Bull.
In the local Gaelic tongue their names would be Each Uisge (pronounced "ech ooshk") for the Water Horse and Tarbh Uisge (pronounced "tarv ooshk") for the Water Bull. No doubt there may have been dialectical variations to these pronunciations (I have seen "ooshkya" instead of "ooshk").
Unlike most cryptozoologists who regard such stories as mythical and unconnected with modern lake monster sightings, this blog takes the opposite view that there is a catalyst for this genre of story that was sightings of strange creatures. That is a theme that will hopefully be developed as time goes on but here are the relevant links below. Any strange or interesting tales from the pre-Nessie era which do not necessarily mention such beasts will also be linked here.
The Water Horses of Loch Ness - link
The Floating Island of Loch Ness - link
The World's Oldest Nessie Document - link
The Elf-Cattle of Caithness - link
Mhorag of Loch Morar - link
The Loch Oich Monster - link (mix of folklore and modern)
The Creature of Loch Ulladale - link (mix of folklore and modern)
The Water Bull of Loch Duntelchaig - link
The Water Bull of Loch Tarff - link
Lewis and Water Horses - link and link
Carmichael Watson Project - link
Fishy Loch Ness Story from 1846 - link
The Loch Ness Kelpie Cartoon - link
The Dornoch Dragon and Nessie - link
Here be Mermaids! - link
The Folklore of An Niseag - link
Thoughts on the Loch Ness Kelpie - link
A Victorian Nessie Story - link
A sighting from 1909 - link
Monsters and Omens - link
Skye Water Horses, Sea Serpents and Dr. MacRae - link
John Keel and the 1896 Loch Ness Monster - link
The Glasgow Evening News and its 1896 Loch Ness Monster - link
The Beast of Loch Achtriochtan - link
The Bell of Saint Cummin - link
The Ancient Serpent Stone of Loch Ness - link