Horse dung/manure on roads is not unlawful but a consideration by horse riders could be the following thus:
A leather type bucket-bag (flat when closed) could be on one side of the horse (behind the rider) with a plastic bin liner in it and a plastic scoop and when safe to do so the rider dismount to scoop up the dung. The dung could be taken home or allowed? to be put in a dog poop bin if one nearby, if one is available,!, ?, the dung already in a bin liner or plastic bag just has dog poop deposits are.
In our village of Cumberworth there are on a section on the road a row of houses and bungalows and some horse riders could "giddy-up" their horse instead of it toileting (urine and dung) very near some front doors of housing, i.e., to allow a horse's toileting where there are spaces between closely nit properties. In summertime where there is horse dung near a front door there is an increase in flies such as large blue-bottles coming off the dung and into near housing. Also, on a damp day the slurry from the dung remaining on the road can be brought into a home on footwear.
Within the High-Way Code horses are not allowed on pavements, public footpaths, causeways but yes on roads, bridal-ways and off-road areas, fields etc.
Kind awareness,
Raymond E.O.Ella.