What is a soul. This is substance that does not require a physical body to exists, nor is it bound to a physical body. The soul can leave the body, as in an Out-Of-Body experience, and it can be disincarnate, in spirit form. Not all living human bodies have incarnate souls. Those humans who are severely retarded, or in a deep coma with scant hope of recovery, or simply in a primitive and self centered, idle, lifestyle may not have souls. On the other hand, they might. It is dependent on what lessons the soul requires, and what school room the soul would best be in to receive these lessons.
Would a soul ever enter and incarnate into a non-human physical form? Most certainly, but only on rare occasions would such an arrangement be allowed. The reason is that this existence does not have much to teach an entity, and this is the consideration by which the birthing envoys are guided. What type of animal might the entity incarnate into? Any kind, although most likely this would be a mammal, a reptile, or a bird. These categories include the mammals of the seas, and small as well as large reptiles and birds. Thus, when you see life, you should consider it to be precious, as you know not what you may be destroying.