Depends on the pony. . . . . . also seen brock around horses, however some horses are mardy little buggers and will have a go at any intruder. So maybe you'll get lucky.
Its a sad fact that you really can't legally do bugger all. They have a daft and pointless level of protection. If it was any other species, including many more useful, beautiful or rare, you could just kill it.
But because its a common, pain in the arse, disease carrying badger . . . . you just have to hope it goes away or walks into a car.