The last thing you want your snark
First (to the best of my knowledge) described by Lewis Carroll
Snark Fit 2: The Bellman's Speech
"For, although common Snarks do no manner of harm,
Yet, I feel it my duty to say,
Some are Boojums--" The Bellman broke off in alarm,
For the Baker had fainted away.
Snark Fit 3: The Baker's Tale
"'If your Snark be a Snark, that is right:
Fetch it home by all means --
you may serve it with greens
And it's handy for striking a light.
"'But oh, beamish nephew, beware of the day,
If your Snark be a Boojum! For then
You will softly and suddenly vanish away,
And will never be met with again!
Snark Fit 8 The Vanishing
"It's a Snark!" was the sound that first came to their ears,
And seemed almost too good to be true.
Then followed a torrent of laughter and cheers:
Then the ominous words "It's a Boo-"
Then, silence. Some fancied they heard in the air
A weary and wandering sigh
Then sounded like "-jum!" but the others declare
It was only a breeze that went by.
They hunted till darkness came on, but they found
Not a button, or feather, or mark,
By which they could tell that they stood on the ground
Where the Baker had met with the Snark.
In the midst of the word he was trying to say,
In the midst of his laughter and glee,
He had softly and suddenly vanished away---
For the Snark *was* a Boojum, you see.
--The Hunting of The Snark, by Lewis Carroll.
Boojums are mysterious creatures -- needless to say, we have no eyewitness accounts. We don't know anything of their appearance or habits (although one might assume that they would be something like snarks; for which, please see the snark node). They are rather like Fnords in this, but perhaps harder to protect against.
Because they are pretty darn scary, and no one really knows what they look like, the word 'boojum' can be used to refer to anything that frightens someone, particularly something imagined or existing only in ones head.
Boojum is also another name for a Cirio* or Candlewood, a large succulent (of the family fouquieriaceae) living on the Baja peninsula. It's basically one big stalk, 40-60 feet tall, with very small branches and leaves. Some are forked, branched, or twisted. Godfrey Sykes (re)named them after Lewis Carroll's snark because of their strange appearance.
* Meaning "Tapering wax candle" in Spanish.