Monsters, by Henry Holiday (left) and J. J. Grandville (right).
[…] One of the first three [illustrations] I had to do was the disappearance of the Baker, and I not unnaturally invented a Boojum. Mr. Dodgson wrote that it was a delightful monster, but that it was inadmissible. All his descriptions of the Boojum were quite unimaginable, and he wanted the creature to remain so. I assented, of course, though reluctant to dismiss what I am still confident is an accurate representation. I hope that some future Darwin in a new Beagle will find the beast, or its remains; if he does, I know he will confirm my drawing. […]
(Henry Holiday (1898): The Snark’s Significance)
Once you meet the Boojum, you might be Going Slightly Mad.