I am always on the look out for centaur stories because I believe they are memories of the first horsemen to an area. I found this one from Scotland...which didn't see a horse until 1000 BC..or therabouts.
Nuckelavee A hideous creature part horse and part man with long sinewy arms. The creature had no skin and its muscle structure and veins could clearly be seen. It had an aversion to fresh water. (The veins could be tattoos)
Kelpies could also be an ancient memory of escaped ponie, hanging around rivers and dangerous. Just a thought