The Kelpie appears to be a stray white pony. However, they are actually a evil water spirit that seek the souls of a men. The Kelpie has no control over man, however it can control a woman and possess her to attract what it wants.
A Kelpie is the antagonist in Seahorsed, a Fan Script by Devilmanozzy.
This is based on actual Kelpie. Read more about them at Wikipedia.