Yes a horse can eat apples.
You can either chop the apple up or give the horse the apple whole.
I feed a friends horse apples when I see the horse at my friends farm and the horse loves apples.
Apples are a very healthy treat for horses and most horses love apples.
I also feed the horse some carrots and pears once in awhile when I'm at the friends farm.
I always bring either some apples, pears, carrots or other vegetables or fruits to feed the horse when I see it.
The horse is always happy to see me with the apples and other fruits and vegetables.
The horses favorite seems to be the apples.