You might know the name from a 30 metre sculpture in Scotland called the Kelpies of 2 horse heads sticking out of the ground.
Now, the Kelpie in folklore is another shapeshifting creature that usually looks like a horse but may appear like a human mostly male but sometimes with or without hooves and of course breathtakingly beautiful. They are said to mostly inhabit the lochs or rivers of Scotland.
The horse is said to have a long wet mane which has waterweeds in them and green seems to be associated with them may they be in horse or human form.
While in Horse form, they would look like a magnificent and tame horse. Tame enough that it looks like you could ride it.
But be warned, you should never ride it because once you’re on top of the Kelpie a sort of adhesive would start to form and you would be stuck on top of it and dragged into the water until you drown where it would devour you.