My horses are allowed into the orchard for a treat and they love to pick up the fallen cooking apples. I do exercise some control about how many they eat but I have never had a problem yet. They also enjoy pears and plums as well as eating apples.
here too never had a problem with apples however crab apples might have given one girlie mild colic this yr but can't confirm this as i was on holiday at the time!
i only let them have half of one per day, per horse. very acidic. this is probably ridiculously over-cautious but it's better than attacks of colic!
i knew a horse which got laminitis from getting into an orchard and gorging on apples...