I agree with Mark and Lauren but if it lasts a lot, call a vet, we never know.
First, massage with cold water, the pressure of the water jet makes blood circulate more easily. Then you can massage with a cream or an
anti-inflammatory gel and apply clay.
If it is hot and soft, it can be a blemish or a bump (maybe the horse banged into something). The best is to slowly shower the leg with cold water twice a day.