One of the cyclists hit her, Jennifer Katherine wrote on Facebook, and “seemed to show no remorse”.
It “smashes along the side of my horse, taking my stirrup in the handlebars and bruising up my ankle”, she added.
The man also appeared to lift his “middle finger at me whilst riding off as I screamed”, she said.
Organisers of the race the cyclists were taking part in said those responsible would be identified and disqualified from future events.
The rider said that as the horse reared up, it lost one of its back shoes.
Jennifer Katherine, who stayed on the animal but was left “in shock”, said no one stopped to see if she was alright.
She said she was “very sore” along one side of her ankle.
In a video posted on the social media site, the horse can be seen walking down a road as bikes taking part in the Windsor Triathlon start to go past.
Initially it is only a few, but then a group passes, including two bikes that undertake the horse as others pass close by on the outside.
Jennifer Katherine exclaims in terror and her mount becomes agitated and turns into the side of the road.
It happened at around 9am on Sunday, she said, and was unexpected because she has lived in the area all her life and her horse is “bombproof on the road”.
There was no warning about the event on the road, she said.
Both she and the horse were wearing hi-vis gear.
“It was terrifying being on a spooked horse and knowing that these individuals weren’t willing to stop,” she said.
From then on, her horse was “on the defence with every cyclist he met”, she added.
Jennifer Katherine said race officials were investigating, and she had had “amazing support from the cyclist and triathlete community”.
The organiser of Windsor Triathlon, Human Race Events, tweeted that it was taking the incident “very seriously”.
“Riders will be identified and disqualified from all Human Race Events,” the company added.
“All riders are briefed to follow the Highway Code.
“We do not condone dangerous cycling of any kind. We sincerely apologise to the individual affected.”