Horses are known for being bad for pasture. Things to look out for are cutting turf up, poaching damp earth, or in very dry weather, standing in same area so much they turn it to dust. Made worse with shod horses. Especially a problem in gateways, by troughs, etc. Also sometimes browsing habits and pooing in same places can end up with some areas full of docks and weeds, some long grasses and some turf neatly nibbled.
Personally, short term, low stocking density, probably won't do too much damage.
If the field is prone to get wet and you do want to top it, think about how much later in the year you could get a tractor on. May be better done sooner while ground is dryish?