Laelaps was a dog that was destined to always catch its prey. He was set by Rhea to guard the infant Zeus on Crete. Later, he was given by Zeus to Europa and then he was given to Minos, Procris, and Cephalos.
Cephalos used Laelaps to hunt down the Teumessian Fox, a creature that was destined to never be caught. Zeus placed the two into the stars so it could play out for eternity in the heavens.