1510: The Beast of Garloll
In 1536, Hector Boece published his History and Chronicles of Scotland, collected from a variety of sources. One of those sources was a man named Duncane Campbell, who told Boece the following story: In the year 1510, a "terrible beist" rose from the waters of a loch called Garloll in Argyll-shire, Scotland. Campbell described it as being as big as a "grew-hound" (a Scottish dog related to the Irish wolfhound) and with the feet of a gander; the beast could strike down great trees with the tip of its tail. Three hunters with their hounds were slain with three strikes of this monster's tail; all the other hunters climbed up strong oak trees, and thus avoided the beast. After the third man was killed, the beast "speedily fled" back to the loch. Campbell then went on to say that a number of men predicted the appearance of the beast foretold great troubles for Scotland as a whole, as the beast had been seen before and trouble had followed that appearance as well.
There is, modernly, no lake called 'Garloll' anywhere in Scotland. However, a Gare Loch (or Gareloch) does exist in Argyll-shire, which is the right area according to the story... so it has generally been assumed this is the loch in question. Gareloch connects to the ocean, so anything walking out of it could have come from pretty much anywhere. And one more thing to note: while the size of the beast was compared to a hound, Campbell did not say the beast looked like a hound. Truthfully, all that is known of its appearance is that it had a tail, and feet like a gander.