One of the robbers disguised himself in a Walrus mask.
At the beginning of the episode "Curiouser and Curiouser", the Raven is robbed by a gang of thieves wearing animal masks as a disguise. The leader wears a grey walrus mask, complete with tusks. The mask covers the front part of his head, including all of the face except the mouth and chin, with holes for the eyes.
The Walrus and the Carpenter, as illustrated by John Tenniel in Through the Looking-Glass, 1871.
Although at the time of the robbery, the particular form of the mask has no especial import, it becomes significant in light of the long dream sequence that comprises most of the episode. In this, Nick is a mortal suffering from delusions of being a vampire in a world that strikingly reverses many of the attributes of his real life. The dream is full of motifs drawn from the works of Lewis Carroll, particularly his two Alice books. In the second of these, Through the Looking-Glass and What Alice Found There (1871), there is a poem entitled, "The Walrus and the Carpenter". Illustrations for the poem show that the head of the Walrus closely resembles the mask worn by the robber at the beginning of "Curiouser and Curiouser". Even though this incident takes place before Nick falls asleep, it should also be interpreted as including one of the Alice references in the episode. Indeed, it is even possible that the similarity of appearance reminded Nick of Carroll's poem and thus prompted his dream.