"It's a heartwarming, wonderful thing to see," said Dr. Bonnie Beaver, a professor at Texas A&M University's College of Veterinary Medicine and executive director of the American College of Veterinary Behaviorists. "These animals generally look to us as if they are very happy. We don't know that they are, but they are excited and jumping around and doing things that are wonderful to watch."
But dogs aren't made to stand, and they are putting unwanted pressure on their joints and probably shortening their lives, Beaver said. Duncan's owners say they are "keeping a vigilant eye" on the animal who balked at a doggie wheelchair and can't use prosthetics.
"Vertical postures are not normal for four-legged creatures," Beaver said. "The use of a prosthetic can give that animal a more normal life."
More veterinarians are using wheelchairs, orthotics and prosthetics to improve the lives of dogs that have lost limbs to deformity, infection or accident, experts say. The move is driven by persistent pet owners who embrace their animals as family and aren't deterred by the cost and commitment of outfitting disabled dogs with the devices. At the same time, there have been great strides in technology to keep up with U.S. soldiers returning wounded from war, and veterinarians have adapted the materials and know-how for the rising demand from clients.
"There are so many things we can do to solve mechanical problems. ... If you have broken parts, we can replace them," said Martin Kaufmann, co-owner of Veterinary Orthotics and Prosthetics in Denver, also known as OrthoPets, which helps about 2,000 animals a year.
Most devices range from $150 to $2,000 but can cost more, Kaufmann said.
Besides commercial manufacturers, there are likely thousands of backyard builders who have created carts, slings or other devices to help their pets get around.
The number of pets using artificial limbs will never be huge, Beaver said. It takes a dedicated person willing to take the prosthetic on and off, clean it and teach the animal to use it. It will likely mean a lifestyle choice for pet owners.
With the time and cost required, many wonder why people spend time on disabled animals when there are so many healthy dogs and cats awaiting homes. The answer, Beaver said, is "some people want to."
No one knows that better than Duncan's owners, Amanda Giese and Gary Walters, co-founders of Panda Paws Rescue for special needs and hospice dogs in Vancouver, Wash. Of the 3,500 dogs they have placed in the last 19 months, 10 were two-legged. Nine of them adapted to wheelchairs and found homes.
Only 19-pound Duncan, whose deformed back legs were amputated, refused to take to wheels. Prosthetic legs aren't an option for him because there are no partial limbs strong enough to attach them to.
The couple often carry Duncan to ease the pressure on his front legs.
"We are his prosthetics," Giese said.
There have seen successes even in challenging cases, Kaufmann said. Orthopets helped mixed-breed puppy Naki'o after his four legs and tail were frozen in ice. What frostbite didn't do, a surgeon did, amputating all four legs. Then, Kaufmann outfitted him with four prosthetics.
"To see Naki'o at the beginning, he was protective and guarded," he said. "Six months after all this was done, he was just a fun-loving guy who likes to socialize."
Now, Naki'o lives with the Nebraska couple that found him.
Another dog teaches kids what it means to be different. The Labrador-golden retriever mix named Pirelli was destined to be a service dog at Canine Assistants, a training school in Atlanta, but one back paw never developed.
Pirelli uses a prosthetic limb, and with it is a "happy, expressive dog, able to run and play, retrieve things and eat his food," said Dudley Arnold, Pirelli's handler.
Together, they have visited about 80 schools.
"The goal was to help kids understand that just because your body doesn't work, doesn't mean you aren't a good person inside," Arnold said.
The rarity of dogs missing limbs varies. Three-legged pooches aren't too uncommon and often make it through life without help. But a dog shouldn't have to do fine on three legs when it can thrive pain-free with a prosthetic, Kaufmann said.
Dogs missing two limbs are still rare across the country, but rescues and prosthetists are seeing more of them.
Giese said she doesn't know what's changed if more dogs are being born without legs or if the animal welfare movement has helped them get turned over to rescues more often.
After Duncan's video went viral, 300 people emailed asking to adopt the dog that has a home with Giese. And her Panda Paws Rescue got a boost in donations and Facebook followers.
"He put a smile on people's faces and was an inspiration to others," she said. "That's quite a lot for a two-legged dog."