The Great Bull of Heaven was a mythological creature from an ancient Sumerian story.
In this story, the Great Bull of Heaven lived in the desert and was killing men by the hundreds. Responding to this crisis, the king Gilgamesh and his friend Enkidu set out from Uruk and fought the beast in its land. As Enkidu held the beast by its tail, Gilgamesh struck it with his sword, achieving victory.
Jean-Luc Picard related the story of the Great Bull to Dathon in 2368 while attempting to find a way to communicate in a manner similar to the Tamarian language, using imagery and mythological references. (TNG: "Darmok")