You will usually need an operator’s licence for your horsebox if you receive money (including prize money) for transporting horses, or if you use your horsebox for professional reasons.
If your horsebox is for personal (i.e. non-commercial) use only, you do not need an operator’s licence.
Remember to check whether your UK driving licence allows you to drive a horsebox. This will depend on the weight of the horsebox and when you passed your driving test. If your horsebox is over 3,500kg (3.5 tonnes), you might need a C1 licence to drive it. You can find more information on Gov.uk.