Komodo dragons are fierce predators, and not animals you’d want to get up close and personal with (if you need convincing, see the National Zoo’s terse description of their hunting technique: “The dragon’s basic strategy is simple: try to smash the quarry to the ground and tear it to pieces”). Castor, a dragon living at the Denver Zoo, didn’t exactly live up to his species' ferocious reputation, though.

In 2009, keepers cleaning Castor’s enclosure noticed that there were hardly any tail tracks in the dirt, indicating that he wasn’t walking around much. They also saw him spending more time in his pool than he used to, moving so little that algae began growing on his scales. When they took x-rays of the 15-year-old lizard, the zoo’s veterinary staff discovered that he had osteoarthritis in both hind legs.

The vets started Castor on a regimen of oral medications—administered by hiding the pills inside smelts and rodents that the lizard ate—but after his condition failed to improve, they decided to take a different approach and called in a specialist.

Tammy Wolfe is a physical therapist with an interesting resume. After two decades of working with people, she turned her attention to animals. She’s certified in canine rehabilitation and runs the The K9 Body Shop, a physical therapy clinic for dogs and cats. She’s also worked with zoos to treat exotic animals like camels, sea lions, flamingoes, coatimundis, antelopes and hawks. Castor—the offspring of a Komodo dragon given to George H.W. Bush as a gift from the president of Indonesia—was her first venomous lizard patient, but she was enthusiastic to help him and began doing regular therapy sessions with him in 2013.

In a new paper on Castor’s treatment, Wolfe and the zoo’s vets describe how they used a therapy technique developed by Wolfe to balance out the workload that the lizard’s different body parts handled when he moved. They started by performing gentle “micromovements” on his spine and pelvis and then added regular massages to their routine, focusing on Castor’s right hindleg.

After a few weekly treatments, Castor’s keepers noted that he was spending less time in the pool and moving around his exhibit more. By the ninth week, he was running (the lizards can reach speeds of 13 mph), and a week later he could climb a 12-inch step in his enclosure, something he hadn’t been able to do for several years.

While the therapy improved Castor’s mobility and quality of life for a while, his keepers noticed this July that he was having severe difficulty moving his back legs and made the decision to euthanize him. He was 21 years old.

“Castor was a remarkable animal and he will be missed,” Brian Aucone, the zoo’s Vice President of Animal Care & Conservation, said in a statement. “Although this is never an easy decision, it was the right one. We’ll all miss him very much, but we’re glad he lived such a long, happy life here at the zoo. It was just his time.”