How did Cancer get its name?
Cancer - "the Crab" (Karkinos in Greek, Cancer in Latin); according to most ancient authorities, the crab was one of Hera's animals. This particular crab was the crab which was unceremoniously crushed by Hercules in his 2nd of 12 labors.
Interestly enough, this constellation was also associated with the term "the Asses' Crib" or "the Asses' Manger." The reason for this association is that in the constellation of Cancer there are two stars named "the Asses" (asini). Most ancient sources agree on these asses being placed in the sky by the Greek god of wine and ecstasy, Dionysus. However, there are several different variations as to what these asses were and why Dionysus placed them in the constellation of Cancer.
One version is that the two gods, Dionysus and Hephaestus, and the satyrs (see Sagittarius) were riding into battle against some "giants." The two gods and satyrs were riding into this battle on the backs of donkeys. The giants, who could not yet see the oncoming entourage, could hear the braying of the donkeys in the distance. For some odd reason, the sound of the donkeys' braying terrified the giants - and so the giants, caught up in a massive panic, fled the scene. I suppose this means that these donkeys were able to make asses out of the giants. By way of honoring of this achievement, the god Dionysus arranged for the donkeys to be placed in the sky as two stars.