charpetrie, Not sure if this will help you but there was some one who owned horses up North a few years ago but I haven't seen or heard of the horses in a while. I believe one of the problems with owning horses up there is getting hay for them and especially good hay from the mainland.
I'm sure there are others on the board who lived closer to these horses who could provide more info.
I heard the horses have died. It's not just lack of food and water. Also access to Vet services. The soil on the island is not good for growing grain products. Everything would have to be shipped from the mainland at a steep price. Have you seen any trails on the north end? It is a dense littoral forest with mangrove swamps intermingled. Not the ideal place to ride a horse. The bugs would devour the riders. Most of the land is either in private or government ownership and horse trails and access would not be available. This is a dream best on the mainland on privately owned property.