"Are horses pets? Does IRS even acknowledge the existence of horses? There is no Principal Agricultural Activity Code for them."

https://www.naics.com/naics-code-description/?code=115210

"This industry comprises establishments primarily engaged in performing support activities related to raising livestock (e.g., cattle, goats, hogs, horses, poultry, sheep). These establishments may perform one or more of the following: (1) breeding services for animals, including companion animals (e.g., cats, dogs, pet birds); (2) pedigree record services; (3) boarding horses; (4) dairy herd improvement activities; (5) livestock spraying; and (6) sheep dipping and shearing."

https://www.naics.com/naics-code-description/?code=112920

"This industry comprises establishments primarily engaged in raising horses, mules, donkeys, and other equines.

Cross-References.

• Establishments primarily engaged in equine boarding are classified in Industry 115210, Support Activities for Animal Production; and
• Equine owners entering horses in racing or other spectator sporting events are classified in U.S. Industry 711219, Other Spectator Sports."

"They all told me that it would be so much simpler to put it on an F, otherwise it is difficult to deduct feed, vet, etc on the Sch C."

We also have to separately report personal property taxes from business use property taxes for ag purposes for the State.

"in an effort to Board Horses as a business."

It helps that they don't have personal stock in those stables, then.

The biggest issue seems to be separating and allocating for their own livestock vs clients' and all of their little side deals. Most times, the person deciding to create and run a boarding facility is already personally committed because of owning and raising their own horses, intending to do riding school or eventing. For instance, they lease the right to use boarded horses in riding classes by giving the horse owner discounts on the boarding costs and the students get a break in class fee if they come in early or stay after to do chores in the barn and paddocks.