First, it may be concerning as to why a horse would be eating magnolia trees. Generally, if horses have adequate, good quality feed otherwise, they do not eat harmful trees/plants.
From available data, magnolia trees are not toxic to horses. Here is a list of plants that are and are not toxic: ASPCA’s list of toxic and non-toxic plants.
However, it is important to note that anything eaten in large enough quantities not normally in a horse’s diet could cause gastrointestinal distress or colic.