The only way I know of to keep them from eating the trees is to fence around the trees, high and wide enough that the horse can’t get to it. It will take some work but your horse will thank you for the natural shade.
I’ve seen places around here that have anything from Chain link to field fence to wrap around the tree. Of course you need to put posts about 2-3 feet from the tree to keep the fence from killing the tree.
You could spray chew stop on them.
You can usually buy it at local feed stores or in tack magazines.
You spray it on and it has a nasty taste and tingly sensation and it usually detours them from chewing the object again.
There are other chew stop type products out there I just cant think of any right off the top of my head.
Just be careful when you use that stuff, dont let it get in your eyes and wear gloves. It feels like you got seven minute itch or something. It goes away in a few minutes so dont be worried, just be warned.
You have identified and know what trees hurt horses so as long as its not those trees it wont make them sick.
My horses used to eat the bark of one tree to close to their pasture and never got sick.
Good Luck!
They will eat bark if the tree is not protected. Most of them know what trees they can eat and what they can’t. You can build a fence around it but make sure it is sturdy. My aunt’s horses have bent ours and ate the tree anyways. We also have an apple tree in their area but they wont take the apples off unless you do it for them and they love their apples, I think too many hit them on the head when they try. Good luck and it should not make them sick.