A: Fruit trees and horses do not mix, especially the types you have listed. First and foremost, black walnut trees contain a toxin that causes laminitis and although it is usually encountered when black walnut shavings are used to bed a stall, the bark on a black walnut tree may still contain enough of this toxin to cause problems in a horse, especially a miniature horse, if ingested.
Secondly, given the size of a miniature horse and the bounty produced by a single apple tree, not to mention multiple apple trees, I would not trust the mini to not gorge himself on apples, gain weight, and develop laminitis from excess carbohydrate ingestion, not to mention develop insulin deficiency from all the fructose. The same goes for figs. Therefore, in a word, no, it is not generally safe to keep a horse in a pasture with trees that drop fruit. Perhaps you could be lenient if there was a herd of twenty horses and a single apple or fig tree, but in your case, the mini is greatly outnumbered by fruit and the risk for developing laminitis, colic, weight issues, and other problems such as choke are just too great to warrant this setup.
To make your pasture safe, ideally, would be to remove all the fruit bearing trees. However, it sounds like this may not be a practical solution in your case, depending on how many trees you actually do have. If there are too many to remove, another option is to erect a fence around each tree. The logistical challenges to this fix depend on how big the trees are; obviously, the fence should be wide enough to encompass any fruit that falls off the branches. Fortunately, the mini’s height works in your favor so the fence could be shorter than if you had a full sized horse. Also keep in mind that the fence needs to have a bar low enough to prevent the mini from sticking his head through the bottom.
Another option to prevent equine access to your fruit trees is to simply fence the trees out from the pasture. Realizing this may not work if the trees are diffusely scattered throughout the pasture, again the mini’s relative size is a benefit, as the mini would not need a pasture as big as a full sized horse, allowing you to possibly fence a reasonable area in the field that does not contain any trees.
One final note on toxic trees: black walnut trees aren’t the only trees out there that are directly toxic to horses. Red maples, cherry, peach, and plum trees all have leaves that produce cyanide when they wilt. Cyanide poisoning is seen in horses that have these trees in the pasture after a storm has knocked branches down and the horses consume the wilted leaves. Black locust trees also contain toxins and acorns from oak trees are harmful as well, if eaten in large quantities.