Horton, 58, spent a decade coaching at Boston College and six years at North Carolina State University. He was diagnosed with Parkinson's disease in 2006 and retired from the football field in 2013. Ten years after his diagnosis, Horton is in hospice care.
Though he wasn't a head coach, numerous players say Horton changed their lives. It's the time he spent with them and how he got to know them that have led some of them to his side as he faces the end of his illness.
"With coach Horton, it was the way he made us feel outside the meeting rooms and practice fields," said former Cleveland Browns offensive lineman Paul Zukauskas. "He was a guy that could be tough as a coach, but he'd ask how your family was doing. He really took the time to get to know everybody."
When Zukauskas, now a high school football coach in Massachusetts, learned of Horton's illness, he called the coach's wife, Maura Horton, to see how he could help.
Zukauskas, Ricky Brown and Al Washington, all former players from Boston College, set up a GoFundMe page
in April to raise money for hospital expenses and an education fund for Horton's two daughters. As of Tuesday, the fund had raised almost $39,000 for the family.
Atlanta Falcons quarterback Matt Ryan, Tampa Bay Buccaneers offensive lineman Gosder Cherilus and former New York Giants defensive end Mathias Kiwanuka are among Horton's former players who have donated to the cause.
"He would be very proud of these young men for helping take care of his girls," Maura Horton said.
What worries Horton's wife most is whether Libby, 13, and Hadley, 8, will know who their father really was.
Hadley has known him only with this disease. It was his diagnosis that drove the Hortons to "complete the vision" of what their family was going to be, his wife said. They sought in vitro fertilization, and Hadley was born.
Their struggle with the disease has also inspired Maura to make the lives of other Parkinson's patients better.
One day, after an NC State game, Horton's limited dexterity prevented him from being able to button his shirt. One of his players, Russell Wilson, now quarterback for the Seattle Seahawks, quietly came over to help the coach.
The story left Horton embarrassed, but it inspired his wife to create MagnaReady
, a line of shirts with magnetic buttons for people with disabilities.
In addition to her work as the MagnaReady CEO and fighting to raise Parkinson's awareness, Maura Horton is fighting to keep the memory of her husband alive for her girls.
"We keep saying over and over we're going to be all right. I'm not letting in, because it'll just be me single parenting. I'm not going to give in," the 46-year-old said between tears.
"They need to be who they are going to be but with the values that Don wanted them to have: education and perseverance."
When Horton went into a hospice home on May 15, family friend Melanie Walker offered a way for the girls to remember their father forever. She asked friends, family and former players to write letters to Horton's daughters, sharing their favorite memories of the coach.
A dozen or so former players sent the family letters describing what the coach meant to each of them.
"In a football world where players are treated as numbers, you treated us as people, and stood out," wrote Ryan Utzler, former running back at Boston College.
One of the most emotional letters came from former Boston College player DuJuan Daniels, a national scout for the New England Patriots.
Daniels wrote about how Horton came to his high school in Indiana to meet him and how he later helped ease the player's cross-country move to North Carolina.
"When I sat down with your dad that day at my school, I knew he was the coach that I wanted to follow," he wrote to Horton's daughters.
Football wasn't the only thing Horton cared about, he added: "He cared about school, he cared about your family, he cared about you, the kid he was welcoming into his family.
"I knew he would look after me, just like he promised my mom, sister and grandmother he would with me being hundreds and hundreds of miles away from the only place I had ever known," Daniels wrote.
Daniels said he felt safe and cared for. Many of the letters noted the same thing and suggested why: Horton believed in his players, or his men, as he called them.
When Daniels suffered a career-ending knee injury years later, Horton was there to support him, he said. The coach offered him career advice and treated him like "part of his family."
Even after they parted ways, Daniels wrote, Horton called him twice a month in the 14 years that passed since his last game at Boston College.
"I can assure you that your dad, Don Horton, had a heart of solid gold. You two, along with your mom, Maura, are forever entrenched in it," Daniels wrote.
"I am happy to call you two my 'little sisters.' "
Horton's wife hopes his legacy will be a lasting one, for their daughters and the world.
"People can make such a difference in such a short time in life," she said. "I feel very protective of the girls, and I want them to know how loved they were and how special he was, to be able to be those kind of human beings that will change the world, too."
If you know Horton or want to share a message, the family asks that you email firstname.lastname@example.org.