Here is one example: Horse Boarding Contracts
One thing you might want to do is make an outline list of what you want to include. For example, the requirement for annual coggins, regular vet care, when board is due, the starting board with a caveat that board may go up with 30, 60, 90 day notice etc. You may wnat to include a timeframe if they are going to pull the horse but if I am reading correctly you arne't really a boarding barn per se..that requirement is more for barns that may have a waiting list for boarders or who want the time to find another boarder so they don't lose board money. Requirement for emergency care if the owner can't be reached...do you or will you have permission to contact the vet if an emergency occurs and the owner can't be reached? Detail out feeding/turnout routine and any requirements for a quarrantine period.
You also want a disclaimer/release form in there as well which should be separate from the boarding agreement which releases you from any liability related to injury to the horse that is not the result of your negligence (ie fences and paddocks are in poor repair or the barn caves in due to poor maintenance) or injury to the owner as a result of them utilizing your property.
As for non-payment, a former barn of mine had a 60 day clause. If board fee was not paid within 60 days, a lien could be placed on the horse until such a time as board was paid, or, if 120 days passes, the barn owner had the right to take the owner to court and sue for possession and ultimate disposition of the horse.
I am embarrased to say I don't even know the non-payment clause for the barn I am at right now. I signed a boarding agreement nearly 7 years ago now and haven't gone back to re-read it. I know I do have a 30-day minimum notification if I wish to leave but otherwise......