Stuffed Alligator
The Stuffed Alligator, or many others just like it, make several appearances throughout the Discworld. It, or their, first appearance was in the Wandering Shop of Skillet, Wang, Yrxle!yt, Bunglestiff, Cwmlad and Patel in The Light Fantastic.
The reptile is indicative of the Wizard in the manner of the barber-surgeon's striped pole or the pawnbroker's balls. It's a standard accessory in their consulting rooms and many keep one in their rooms at Unseen University. Why alligators, specifically, hasn't been explained and crocodiles may be substituted on the basis that few know the difference.