Mark Todd (NZL eventer) and John Whitaker (GB showjumper) have their own brands that make and sell clothing for riders and equipment.
Lots of equestrian sports people have major sponsorship deals, quite a few that aren't necessarily horsey brands too for example:
Mary King (GB Eventer) had a long running deal with Joules and had her own range with her name all over items from it.
H&M have deals with several showjumpers, (Olympic Gold medalist Peder Fredricson, and I think three others) and even have their brand name in the name of an Olympic Gold Medal winning horse - H&M All In.
William Fox-Pitt has a deal with Jeep, and also Musto. Harry Meade is sponsored by Land Rover.
John Whitaker has over his long career had sponsorship with all kinds of companies including Jaguar Landrover and Next.
Most riders need sponsorship to survive, the mainstream press aren't interested in many of them though so outside horsey circles people don't hear of them. The only other rider likely to get anywhere near the interest is Charlotte DuJardin she lists 20 sponsors on her website but most non-horsey people wouldn't have heard of them.