According to Kinbote (in VN’s novel Pale Fire, 1962, Shade’s mad commentator who imagines that he is Charles the Beloved, the last self-exiled king of Zembla), Gradus (Shade’s murderer) is a cross between bat and crab:
The grotesque figure of Gradus, a cross between bat and crab, was not much odder than many other Shadows, such as, for example, Nodo, Odon's epileptic half-brother who cheated at cards, or a mad Mandevil who had lost a leg in trying to make anti-matter. (note to Line 171)