According to Osho, the serpent is the most fitting symbol for kundalini because it mirrors its qualities: a legless, pure energy that moves within; coiled dormancy at the base unwinding as it rises; completion as a self-circling circuit (tail-in-mouth); expansive hood like the flowering at sahasrar; an erect rise without physical support; and its innocence, dangerous if mishandled, yet wise, alert, and potent.
We use a snake for kundalini because it sleeps curled up, then uncoils and rises by its own energy, and is gentle yet powerful—just like our inner life-force.