A wombat is a small mammal somewhat like a kangaroo.
To quote Ogden Nash,
'The wombat lives across the seas
Among the far Antipodes.
He may subsist on nuts and berries
Or, then again, on missionaries.
I, for one, would not engage the wombat
in any form of mortal combat.'

The Jargon File mentions wombats too. . . it stands for Waste of Money, Brains, and Time, for example, "Running any Microsoft program for Unix is a total wombat."