Section 9 of the Animal Welfare Act 2006 (legislation.gov.uk) (AWA) requires you to ensure that any horse, pony, donkey or mule for which you are responsible, whether on a permanent or a temporary basis:
To meet equine behavioural and physical needs the environment in which equines are kept should allow for the expression of their natural behavioural repertoire (NBR). This includes the provision of company and the opportunity for social interaction, sufficient space to allow the animal to move around freely and adequate forage.
Species-specific and individual differences should be considered when making provision for the animal in your care:
For information on mules see What is a mule? | The Donkey Sanctuary.
The British Horse Society provide information on horse (and pony) care and management (Horse Care & Management | Advice & Information | The BHS), including specific guidance on stable management and safety (Stable Management and Safety | British Horse Society (bhs.org.uk)), and how environmental enrichment can be introduced to improve equine quality of life (Environmental Enrichment for the Horse | The Need for Companionship (bhs.org.uk); Environmental Enrichment for the Horse | The Need to Forage (bhs.org.uk); bluecross.org.uk/pet-advice/horse-enrichment-activities.
For information on the management of all equines and guidance for compliance with the AWA see the Code of practice for the welfare of horses, ponies, donkeys and their hybrids (publishing.service.gov.uk).
Horses generally require a minimum of one to two acres per horse. This can vary depending on the size of the horse, the type of land, the quality of the grazing and on the type of pasture management practised. Yards and pastures should be properly fenced to confine horses. The ideal fence is post and rail with the rails set on the horse side of the fence. Gates should be kept closed and preferably locked.
For information about managing pasture for horses see Pasture management for horse paddocks - World Horse Welfare and Horse Pasture Management | Advice & Information | The BHS.
Plants poisonous to equines should be removed from the pasture.
Information on specific aspects of equine management can be found via the links below: