GREENWICH — Mark Fontana was a typical toddler until, at age 3, he began losing his words and skills, gradually not making eye contact or interacting with the world around him as he always had.

“It was like someone erased him,” said his mom, Emily.

Doctors diagnosed Mark with late-onset regressive autism, a shattering blow to his family of seven. But the youngster’s parents vowed to leave no stone unturned to help Mark.

That’s how they found Pegasus Therapeutic Riding, a program that serves people with disabilities and challenges through horseback riding and other equine-assisted activities and therapies. Founded in 1975 in Brewster, N.Y., the nonprofit organization has a regional chapter at Greenwich’s picturesque Kelsey Farm on Lake Avenue.

It’s there that Mark, who will be 10 in September, has spent two years in Monday classes, developing a newfound calm that counters the internal and external triggers that often spur unpredictable behavior.

“I can’t say enough wonderful things about Pegasus,” said Emily Fontana. “They just know what they’re doing.”

Pegasus instructors work with students on the autism spectrum; people with cerebral palsy, Parkinson’s disease and Down syndrome; those who use wheelchairs and braces, individuals who have experienced a stroke or other condition that creates muscle weakness; and others, said certified instructor Victoria Fullerton.

Fullerton, who has a physical disability, learned about equine-assisted therapy when her family sought it out to help her strengthen her affected legs and hands.

“I found a passion for it,” said the Yonkers, N.Y. resident, who is earning a graduate degree in occupational therapy. “I felt the horses heal me. Today I feel very strong.”

Pegasus (www.pegasustr.org) bases its private and group sessions on the known benefits of transferring movement from a horse to a rider.

A horse’s movement closely resembles the normal gait of a human. Riding a horse — with direction from a trained instructor and assistance from leaders and “side walkers” who ensure safety — can benefit people with sensory-integration issues. For instance, the movement might provide input needed to help a rider establish rhythm or give the stimulation needed to help organize and integrate sensory input.

Therapeutic riding can also improve mobility, balance, posture, coordination, language development, behavior control and concentration.

Fullerton, who usually works with children at Pegasus, said the beauty of the program is watching the progress each student makes and the unique bond they forge with the horses.

“I’m not the therapist,” she said. “The horse is the therapist.”

Each student — and they range in age from 4 to 80 — is evaluated and given a program customized to his or her particular situation. For instance, a non-verbal child might be taught how to communicate commands to the horse through gentle tapping on its back.

A student who seems agitated or fearful might walk with the horse instead of riding that day.

“We meet them with whatever they bring that day,” Fullerton said. “You just make it work.”

One technique that seems to make a difference with Mark Fontana is backwards riding. Some students require regulation of their neurological system, Fullerton said. Seating the rider backwards and laying his or her arms along the horse’s body gives the student more input from the horse, providing a calming influence.

“They can read body language so well,” Fullerton said of the horses. “The horses seem very in tune with it.”

Emily Fontana has seen the benefits for her son, who has been working with a horse named Jack. At first the Rye, N.Y., mom worried that the instructors would eventually ask Mark to leave because he is prone to screaming and impulsive behavior.

“That day never came,” she said. “Jack is unflappable. And the staff are so patient and aware of the riders. They always talk about potential, not what someone couldn’t do.”

That positive spirit is one of the reasons Greenwich resident Trish White has been volunteering at Pegasus for more than five years. A metalsmith and jewelry artist, she’s at Kelsey Farm for about five hours each Monday during the spring and fall sessions.

While she enjoys interacting with the horses and riders, she also grooms and prepares the horses and has mucked out more than one stall.

“I wouldn’t miss it for the world,” said White, who recently received Pegasus’ 2018 Pat Bugg Award for Special Service and Dedication. “It’s all about the horse. It’s such a great therapy and you can see the changes in the riders. You can see them get stronger in their core.”

Fontana continues to look for new ways for Mark to engage with his world. They have enjoyed dolphin therapy in the Bahamas, and the family welcomed Echo, an autism service dog, to their home a few years ago.

“I’m convinced at some point science is going to catch up — It might not be in my lifetime,” she said. “Until then, the small things aren’t small.”