كدلالة صهيل الفرس على طلب الماء والكلأ
As a horse's neighing indicates a desire for water and fodder.
A neigh is not an articulated word. The textbook treats it as a natural expression that may reveal an animal's need; hence it is placed under nonverbal natural signification.
A horse can neigh in other situations too, so one should not infer hunger or thirst from every neigh. The example explains the kind of natural connection, not a guaranteed diagnosis.