Horse riders are by far the worst.
(1) They think they own the road, and act like it. Usually middle aged, posh ****s.
(2) The animals crap all over the road. Health hazard as well as traffic hazard too.
(3) Horses are extremely unpredictable. You cannot honk your horn either to warn riders, due to this fact.
(4) Horse riders generally have poorer control over their animal than cyclists do over their bikes (duhh).
(5) the list goes on...
Once had a horse rider appear out of nowhere on an A road crossing and began to cross, the stupid woman just expected the driver of the car I was in to stop immediately, despite being only several meters away whilst travelling at around 30 mph. Never longed so badly before to smack some common sense into someone.
Horses should be banned off the road completely (except in police/tactical situations), its the 21st century, not the middle ages.