Chapter 18 Page 79

The form a person's Morphus takes is supposed to reflect and support their role in life - Zharus Guard companions will commonly become the sorts of powerful predators like wolves or big cats or birds of prey you've seen among the palace guard. Snakes are very rare, and some might assume that they and their companions are meant for more clandestine purposes.
That, and some people just really don't like snakes.