A week before the Kentucky Derby, Sandman was a 12-1 choice at some Las Vegas casinos to win the 151st Run for the Roses. By race time, the odds had dropped drastically, to 5-1, and a TikTok star, Griffin Johnson, probably had much to do with it.

The social media influencer is a minority owner of Sandman, and Mr. Johnson’s 9.7 million followers likely wagered a few dollars on the gray 3-year-old, who finished seventh in the Derby. Sandman is entered in Saturday’s Preakness at Pimlico Race Track in Baltimore.

“The social media wave is showing people there’s more than just having a multimillion-dollar upbringing to get you into this sport,” Mr. Johnson said recently. “My biggest passion is getting my generation involved. Getting a piece of Sandman was a gift from God, saying, ‘Hey, it’s time to bring young people to this sport.’ It just needs to be presented to them in a way that is obtainable and digestible.”

Mr. Johnson, 26, is doing that through his popular social media posts, which now frequently feature Sandman. He has flooded his site with videos of his interaction with the son of Tapit, whether it’s petting him, feeding him, or bathing him with a sponge. 

A recent short video of Mr. Johnson feeding Sandman carrots and peppermints earned 253,000 likes. A video from the day of the Kentucky Derby had 3.4 million views.

“People want to see everything about horse racing, from what they eat to how they’re bathed to where they sleep,” Mr. Johnson said. “It’s really incredible how interested people are in the A to Z care of these horses. We’re collaborating with Gen Z and some of the younger millennials that are on TikTok. I want to be as useful as possible to the sport.”

The Preakness is the second race in the Triple Crown and will feature a nine-horse field, only three of which ran in the Kentucky Derby. Sovereignty, the Derby winner, will rest until the Belmont, eliminating any chance for a Triple Crown winner in 2025.  American Promise and Journalism are the only other two Derby horses at Pimlico.  

Journalism, runner-up in the Derby, is the betting favorite to win the 2025 Preakness, while Sandman is second at 4-1.  There is a 60 percent chance of rain, and a wet track would be similar to the conditions two weeks ago at Churchill Downs.

Rain isn’t going to dampen Mr. Johnson’s mood. He’s thrilled to be part of the horse racing scene after connecting with West Point Thoroughbreds, a partnership company with an initiative to get influencers and celebrities involved as owners. It was love at first sight when he was introduced to Sandman.

“I saw him and was sold immediately,” Mr. Johnson said. “It was kind of a ‘I trust you, you trust me’ moment. No matter how much money you spend or how much prep you put into it, you never know if you’re going to get a true winner. I just thank my lucky stars I got a piece of Sandman out of all the horses.”

A native of central Illinois, Mr. Johnson grew up three hours from Churchill Downs and gained TikTok fame by posting videos of his journey through the medical field. That led him to move to Los Angeles and expand his brand into acting and producing. He also has a venture capital company.

He’s not just along for the ride when it comes to horse racing. Among his goals are to improve aftercare for horses and spread awareness among the younger generation.  “My biggest passion is getting my generation involved,” Mr. Johnson said. “If you take a blend of social media and traditional entertainment and find synergy, it can give people a new perspective. The industry is starting to recognize that in order to expand, it’s going to have to open its arms and show people what the sport has to offer.”

Sandman’s Derby trip was spoiled by a crowded start in the 19-horse field. He dropped far back before rallying on the muddy track to finish seventh. “He obviously wasn’t handling the track very well,” Sandman’s Hall of Fame trainer, Mark Casse, said, “and you just don’t know how that affects them late in the race.”

Initially, Mr. Casse thought he might skip the Preakness and rest Sandman, but he quickly changed his mind. “We were planning on waiting for the Belmont,” Mr. Casse said. “But a lot of things can happen in between. He’s telling us he’s happy, so we’re going to go with it.”

Mr. Johnson plans to chronicle every moment leading up to the Preakness for his nearly 10 million followers. “I feel for everyone who has put their heart and soul into this sport for so many years,” he said. “There’s people who have been in this their whole life, and then there’s me.”