As far as I'm aware a landowner that has animals on his land without permission has to serve notice that the animals must be removed, if the animals are still there after 14 days the landowner can claim the animal and keep, sell or slaughter as required.
Steps to take under the Animals Act 1971 – (as amended by Control of Horses Act 2015)
Even if you know or think you know who the owner is, you must notify the officer in charge of a police station and obtain an incident number. You may wish to put up an abandonment notice for a minimum of four working days (England) although this is not a legal requirement. Download a sample Abandonment Notice here.
A responsible owner should attend to their horse at least once a day, so putting a notice in place for four days would be more than adequate warning for them to respond.
If you do place a notice it should include a contact number and address, a very short description of the horse and your intention to remove the horse(s) if no owner comes forward. Display the notice prominently where you found the horse(s).
It may be worth using a sign even if you think your land is being fly-grazed – you may find the horses will disappear.
Fly grazing guidance - image 2 June2016.jpg
If an owner comes forward
If you are approached by someone claiming to be the owner, it is advisable to ask to see the horse’s passport, which they should, by law, have in their possession. If the horse was born after 2009 the passport should include a microchip number; this number can then be verified by scanning the horse with a microchip scanner. This will prevent someone fraudulently claiming the horse. The passport will contain a detailed description of the horse.
Remember that a passport is not legal proof of ownership, however it should give some assurance that the person is the owner. There may be situations where a person may not have a passport to show, such as when they have only just purchased the horse. Alternatively if someone has photos or can give an accurate description this should be acceptable as an indication of ownership. If you have concerns that they are not the legal owner of the horse you can contact the police for advice.
You do not have to release the horse until the owner has reimbursed you for any damage caused by the horse to your property and any expenses reasonably incurred in keeping the horse an ascertaining who owns it. If the owner refuses to pay your reasonable expenses within the “defined period”, you can then rehome, sell, euthanase or keep the horse.
The “defined period” is set out in the Animals Act 1971 and is 96 hours from the period when the horse was first detained but ignoring any time falling on any of the following days: 1) Saturday or Sunday, 2) Good Friday or Christmas Day, 3) A day that is a bank holiday in England and Wales (as defined by the Banking and Financial Dealing Act 1971).