Triathlon organisers are investigating as a pony was make and its rider hurt by cyclists racing too close to them.
Footage posted on Facebook by the rider showed the moment she said she and her horse were hit during the Windsor Triathlon at about 09:00 BST.
The video proves cyclists pass on both sides, the pony react to a jolt, and the rider cry out.
Organiser Human Race Events said anyone found responsible would be disqualified from the race.
The rider, posting under the name “Jennifer Katherine”, said both she and the horse were wearing high-visibility garments at the time of the crash.
She wrote: “The cyclist smashes along the side of my horse, taking my stirrup in the handlebars and bruising up my ankle.
“My horse rears and bolts forward, taking off one of his back shoes.”
She said she had been unaware the event had been planned.
“No-one stopped to see if I or the pony was OK after this hit, ” she said.
“I’m still in shock and feeling very sore along one side of my ankle.”
The Windsor Triathlon has been running for 28 years and challengers start and finish the race near Windsor Castle.
Amanda Coyne, who lives near the route of the race, said she had encountered cyclists riding “dangerously” while driving shortly before 09:00.
“I virtually had a heart attack as I nearly got hit by a vehicle which had been forced to overtake cyclists who were riding four-abreast, ” she said.
“They were riding at such velocities and didn’t care who was coming in front of them and who was behind them and I really worried there was going to be a terrible accident.”
She said she stopped to speak to a race official who she said rejected her fears.