If a horse has a colic history, is it okay to have an apple tree in it's pasture?

It depends on what has caused the colic in the past. I doubt it was apples. Any risk posed by an apple tree in the pasture is very small. My pasture has several apple trees in it, and the only problem I've had with them in the past 10 years is when my one pony got old and his teeth went bad, he choked once from trying to eat one. That problem was solved by having him wear his grazing muzzle for the couple of weeks when the apples fall off the trees. The rest of the year they're just normal trees, and the horses leave them alone.