1889 (pub): The Thieving Kelpie
A tale is told of a kelpie who had taken a liking for a woman near the town of Braemar, Scotland, by the River Dee. The woman lived near the mill of Quoich... despite this, she had run out of meal and was unable to get what she needed. The kelpie that had fallen for the woman decided to help her in her plight without being asked... and so one night when he knew the mill was grinding corn, the kelpie went to it after the miller had left. Because the meal was very slow to do its grinding, the miller was in the habit of filling the hopper to the top so the mill would have plenty to grind all night until the miller returned in the morning. Knowing this, the kelpie entered the mill and waited for the sack that received the ground meal to fill. Once the bag was ready, the kelpie lifted it on his back and head out for the woman house; but by this time the sun was just rising, and the miller was on his way back to the mill to make sure everything was going okay. When the miller saw a tall man coming around the corner of the mill carrying a sackful of meal, he was not happy! The miller grabbed up a "fairy-whorl" that was sitting at one of the mill's corners -- a stone normally used at night to keep the mill from turning -- and the miller hurled it at the strange man while shouting out "kelpie or nae kelpie, God damn you, a'll brack your leg!" The stone must have heard this, for it most certainly did strike the kelpie's leg and break it... the kelpie tried to escape but tumbled into the "mill-lead" where the torrent of water from the mill-wheel carried the poor beast off into the River Dee and drowned it.
Source's Sources & How to Drown a Water-Monster
The tale above was first published by the Reverend W. Gregor in 1889; he had collected the story orally from a Mr. J. Farquharson, a mason in Corgarff, who, in turn, had heard the tale from a man named D. McHardy in Ardjerige... so this tale made the rounds!
So how exactly does it make sense that the kelpie -- a well-known water monster in Scotland -- could be killed by drowning? This tale seems to build on an assumption that can be seen in other Scottish tales: that if a supernatural creature should take a human or animal shape to interact with people, then that supernatural creature is also making itself vulnerable to the same sort of things that would be dangerous to a human or animal. So, in human form, the kelpie could not handle its natural element any better than an injured human could... leading to an ironic conclusion.