It was a Boojum
[lower left]: Allegorical English School painting (ca. 1610) as above: Mirror view, converted to black&white (in order to focus on shapes).
[right]: Allegorical English School painting (ca. 1610) as above: Some segments rearranged in an attempt to understand Henry Holiday's way to quote from this painting. Into the large image on the right side I inserted the mirror view of the Bellman's face (from the front cover of Carroll's poem).
The pictorial quotations by Henry Holiday in his illustrations may help to interpret The Hunting of the Snark. I think, Holiday understood Carroll well. And Carroll seemingly got afraid a bit that his poem was too dark: "And if I have written anything to add to those stores of innocent and healthy amusement that are laid up in books for the children I love so well, it is surely something I may hope to look back upon without shame and sorrow (as how much of life must then be recalled!) when my turn comes to walk through the valley of shadows." (Carroll's Easter Greeting, 1876)