I owned a Tennessee Walking Horse at one time. I did not hire him out when I first bought it. He had been abused, and neglected and was quite jumpy. I did not ride him myself when I first bought him. I concentrated on getting rid of the horrible thrush he had in all four feet, and getting him to calm down and trust me. I had a second job at the stables to support him, and that gave me plenty of time to be around him.
After he settled down, the trainer started riding him, and I got to after a while. Still, no one else could, though he enjoyed the company of just about everybody. But, a stray brush of a pant leg would flatten his ears, and he'd take off full speed, out of control. The scars on his back side provided ample evidence of why
After my trainer and I had put a bunch of work into him, she started having more advanced riders work with him. He eventually became as easy to ride as he was to handle in every other respect. Poor guy just wanted to make you happy, but he wouldn't pick his front feet up to show standards and had gotten clobbered for it. He just wasn't a show horse, but there wasn't a finer pleasure horse to be had.
At that point, I would loan him out to riders the trainer knew would be gentle with him. No beginners (besides me!), because he really didn't like the conflicting signals they give. He needed to know what you wanted without a lot ambiguity.
I did not lease or rent him out. I only loaned him out because he loved to be ridden, the more the better. I was not willing to let just anyone ride him. I knew not just the riding capabilities of the people I let ride him, but their personalities as well. Somebody who's only focus was on winning shows never got on my horse.
If you are going to let a trainer lease your horse out to offset boarding costs, make sure you know and trust your trainer. Most renters take better care of horses than they do planes, but not all. Plus, the horse is a living creature, and some just aren't suited to being ridden by a bunch of different people. You've got to know your horse and trainer very well. If there are problems and questions before you put the horse in their hands, I would make sure they are resolved before trusting my horse to them.
I miss Spur. He was a great horse, but getting engaged took the time I needed to work at the barn. He was sold to a couple who take him trail riding on a nearly daily basis, and has a perfect home for him. I made a huge profit on the sale, but would rather have the horse and the wife.
There are times I think I'd be willing to give up flying for horse ownership. Hopefully, I have more money when Sean is 5 or 6 years older, or I might have to.