Most of them are crabapples, planted for their flowers and "fruit" such as it is.
While it's possible for one of the horses to consume crabapples or the decorative maple or weeping wild cherry trees, it's not common, and usually happens either in periods of drought/famine or incidental (as where someone mows a lawn mulching up leaves with grass and then spreads the grass or dumps it in a pile somewhere.
Apples of any variety can cause upset stomach in horses and people too) in small quantities and, if eaten whole, can be toxic. This is amplified with crabapples because they are smaller and the flesh/fiber is proportionately less than the seed content, which is problematic for animals and humans because they contain cyanogenic glycosides that metabolize into the toxin cyanide.
Crab apples is an umbrella terms based on size. They aren't actually a different tree species, and there are many species of crab apples. Their small size makes most of them quite bitter and unpalatable, so while a horse might eat them incidentally, they would not usually seek them out.
Usually what you'll find people have done is scattered these or other apples in the sand with carrots and greens to attract the horses to an area so they can get a photo op. Most people really don't appreciate the danger to these horses in particular, who have little variation in their diet, or to other horses in general.
It's highly unlikely a colt would seek them out, or that a mare would bring her foal that close to these trees as horses normally avoid them and the foal itself would be young enough to still be nursing. Nursing horses don't eat much grasses, let alone scavenge for novelty items.