You can read more about limericks at Wikipedia, and you can find some wonderful examples in the UnTextbook too.
The Aesop's Fables unit has the fables in limerick form, along with illustrations by Walter Crane. So, for example, here is the famous story of the "sour grapes" and the fox:
This Fox has a longing for grapes:
He jumps, but the bunch still escapes.
So he goes away sour,
And, 'tis said, to this hour
Declares that he's no taste for grapes.
There are also some limericks in the Nursery Rhymes unit, like the Old Man of Bombay:
THERE was a fat man of Bombay,
Who was smoking one sunshiny day,
When a bird, called a snipe,
Flew away with his pipe,
Which vexed the fat man of Bombay.