Limerick
a humorous poem five lines long in which the first, second, and fifth lines have one rhyme and the third and fourth another.
'Humpty Dumpty' is not a limerick. Limericks all have five lines; 'Humpty Dumpty' has just four. Limericks have a specific rhyme scheme: AABBA. 'Humpty Dumpty's rhyme scheme is AABB.
Edward Lear
“There was an Old Man with a beard, Who said, 'It is just as I feared! Two Owls and a Hen, Four Larks and a Wren, Have all built their nests in my beard!' ”