Kelpies

I’ve always had a fascination for Kelpies, the sinister waterhorse of Scottish folklore. Deceiving weary travellers by appearing as a beautiful black horse and tempting them to ride, only to turn into a demonic creature as soon as the humans are on their back. Mane turning into writhing tendrils which wrap around their victim, before diving into the nearby loch or river to drown their victims in the depths.

However if you really want to scare the wits out of folk this Halloween, then you want the Nuckelavee possibly one of the most terrifying of demonic water creatures.

http://www.orkneyjar.com/folklore/nuckle.htm