as the other poster said, unless the horse is being starved or badly injured the animal rescue wont touch them as they are overflowing with horses. we had a similar problem with a neighbour, who had an uncared for horse, but since he occasionally threw it hay and water the rescue said they couldnt do anything.
I know legislation in certain areas is different. i believe waterford i think it is, impound any horses found on estates.
The other poster mentioned how it might effect you. as a horse owner and lover myself, they come with a huge amount of smells and invariably a muck heap that builds up fast. Plus an unhappy isolated horse will usually do anything to escape and can barge through gates and try to jump walls etc. it was very distressing watching our neighbours horse doing this, it used to end up wandering on roads and breaking through other peoples fences.
saying that though i know a case of a horse lover who couldnt afford livery and only had the choice of keeping it in her estate garden. the horse was well cared for and properly maintained, and taken out to exercise, graze etc. this was only a temporary measure till she got a new job. its completely unfair to keep a horse in that enclosed space. they should have access to graze freely for as long as possible every day, and also another horse or goat for company as they are social animals.
if you say theres a problem with horses on your estate though id imagine it might be some kid who hasnt a clue. could you find out more about their circumstances. our neighbour eventually moved his horse due to numerous complaints from neighbours.