Also, if horses are purely grazing the land and not being fed additional feed or exercised (ridden or driven) then altho horses (excl working horses) aren't agricultural animals in planning terms that use of the land IS considered agricultural and change of use is not required.
Grey area if hay is put out for them, that might breach the exemption, but then if the other livestock is out there too the hay could be for them.....
Any structures or equipment solely for the horses on the land would undermine being able to use that exemption, I think tho.