Frank J. Wilstach, comp. A Dictionary of Similes. 1916.

Unlikely

Unlikely as that a mouse should fall in love with a cat.
—Anonymous

Unlikely as that the wolf is to eat the moon.
—Anonymous

Unlikely as sweet fruit plucked from a dry tree, or sweet leaves on a dead stem.
—Anonymous

Unlikely … as to teach an alligator the polka.
—Anonymous

Unlikely as that a moth intends to be burnt when it flies into a candle flame.
—Anonymous

Unlikely as to see a hog fly.
—Anonymous