You should be able to look it up on line with a google search; although I'm sure someone will come on sooner or later and let you know.
I haven't heard of too many issues with apple, but you'd want to watch the dropped apples to make sure your horses don't get too many and cause a bellyache. [Pick up all fallen apples or fence the horses away from the apple trees.]
Black walnut will cause founder if your horse steps on any part of the tree (leaves nuts or twigs on the ground) even shavings from black walnut wil cause founder as they leech toxins. Cherries produce arsenic so they are no good, and choke cherry is poisonous only if the leaves are wilted, not fresh or dried, so watch for recently broken branches. Apple trees are not toxic, but horses will founder from eating too many apples and they can cause gas colic. Crabapple trees will drop thosr sour apples on the ground and the fruit will ferment ive seen horses get drunk from eating too many.
I'm not sure about all types of cherry, but I do know tha wild choke cherry leavges turn to arsenic when wilted and are toxic. On apple trees, if your horse eats too many apples with the seeds, the seeds turn to arsenic can cause colic or toxic event.
Choke cherries are dangerous, leaves, bark or wood, I know a year or so ago someone on here lost 3 horses because they had cut wild cherry in the pasture and the horses ate the sawdust and chewed on the trunk....black walnut is most recognized for causing founder but can cause all kinds of issues, oaks...acorns can be very dangerous as well, and red maple is known to be toxic...also yew and poinsettia bushes.
Red Maples and Sugar Maples are definitely toxic! Two of our minis survived thanks to intensive ICU care at UC Davis Vet Hospital. Sadly, three weeks earlier, a sweet percheron draft passed due to toxicity poisoning although the cause and source of the poisoning was unknown at the time. Discovered that it was due to sugar maples. We were informed that many varieties of maples are hybrids of red maples. Also studies have indicated that pistachios and walnuts are highly toxic to equines. Although less toxic than others, oak leaves and all acorns can also be toxic and deadly to miniature equine.