I'm going to make a Jungian jump here :P

The sea horse may be a personification of your anima self - the part of your psyche that is both more primal and more of the opposite gender of what you are in the waking world. Based on the fact that this is a sea horse, I'd make the guess that it relates to a more serene part of yourself (relating to the fact that seahorses are seen as non-violent, and the fact they live in water). That your sea horse went to like in the exotic locale with your mother, it could reflect that your serene side connects with some underlying part of your mother that you're reading into without actually knowing (hence the exotic location).