"the equipment and the horse and trailer all started to take place in 2023"

It's called "placed in service." Your ProSeries tax prep program has a place to handle assets.

"what kind of deduction can you take? Then there is the  matter of boarding the horse.  Is any of that deductible?"

How is your taxpayer being paid? On a callout basis as a contractor, or as an employee? What sort of payouts are there for these required items? What else are these items used for: gymkana, gymkana or county fair security? Riding lessons (at the boarding facility)? There should be a handout that helps you know this, from the department.

Read here, as a place to start:

https://ttlc.intuit.com/community/tax-credits-deductions/discussion/reserve-mounted-horse-police-off...