In 2008, when his new family adopted him, Dozer the Goldendoodle was the only pup left in the litter.
"He was the last of the bunch," said Rosana Dorsett, Dozer's owner. "He was the dog no one wanted ... but he's got a great heart."
Watch "World News With Diane Sawyer" for more on this story tonight on ABC.
It made Dozer kind of an underdog. But fast forward three years to the day of the Maryland Half Marathon -- a 13-mile race for cancer research -- and this pup found his way to the front of the pack.
That was the day Dozer slipped past the virtual fence surrounding his yard as the marathon runners passed by. He got quickly caught up in the current at the 5-mile mark -- and kept up the pace for the remainder of the race, with people snapping his picture all along.
When he crossed the finish line, the bewildered pup with muddy paws turned to walk the eight miles back home, where he was awarded a finisher's medal from the University of Maryland Marlene and Stewart Greenebaum Cancer Center.
Suddenly Dozer's life story changed. A Facebook page was put together in his name to raise money for cancer research. Donations came pouring in, as did the fans. He now has 2,500 friends on the social media site and has raised more than $17,000.
To join Dozer's cause, click here
Supporters and survivors post on the page daily to thank Dozer for joining the marathon.
"As a cancer survivor, I'd just like to thank you for all you are doing to help," one follower wrote.
"You are our hero! We 'wuff' you," wrote another.
Michael Chaykowski, a disabled retiree in Florida -- saw Dozer's page and decided to donate, too, but for another reason.
"At first I laughed. I thought it was really funny ... after watching the video I started to cry, saw it was more than just a joke," he said. "He's been a great inspiration for me. I suppose, most of all, he's taught me how to believe in myself.
"Dozer is America's dog, inspiration for us all," said Chaykowski.
While the wonder dog seems to inspire all and gives laughs to some, his run has benefited others in the best way of all.
Diane Salvatore, 55, was just diagnosed with triple negative breast cancer. She is a direct beneficiary of Dozer's fundraising -- the more than $17,000 that the pup raised has already been designated to go toward research that will help her and others like her.
"I don't think he's last anymore," said Salvatore. "I think he's come in first place. Great job to be the spokesdog for this type of research that needs to be done for this kind of breast cancer."