The asp, a snake with eviscerating venom, will put one ear to the ground and stop the other with its tail to avoid hearing the snake charmer’s spell. In most of these images, the charmer reads his spell from a scroll or book. Though a snake, the asp seems to have acquired legs and wings in several images.
In the Aberdeen Bestiary (Aberdeen University Library MS 24), the asp is further sorted into six species, which you can read more about here.
The moral lesson the asp was meant to teach was that of the evils of too worldly men. Like the wealthy or greedy, the asp has one ear stopped up with earthly desire, and the other with the sins of his deeds. Because of this, he cannot hear the words of the good or of the lord—”The wicked have been estranged from the womb, They have erred from the belly, speaking lies. Their poison [is] as poison of a serpent, As a deaf asp shutting its ear, Which hearkeneth not to the voice of whisperers, A charmer of charms most skillful.” (Psalms 58:3-5)