Back when I was an undergraduate, one of my professors explained a theory about why bulls were important to the prehistoric people (and they were obviously important, since bulls are depicted in so many prehistoric caves. This example on the left comes from Lascaux Cave in France (c. 15,000 BCE)).
The theory presented by my professor revolves around the St. Elmo's Fire phenomenon. Basically, sometimes during electrical weather storms (i.e. storms with thunder and lightning), the tip of a bull's horns can have a soft glow. The glow often is accompanied with a hissing or crackling sound.
It is thought that this phenomenon would have impressed prehistoric people, which may account for the supposed veneration of the bull. It could have been seen as a mystical creature with supernatural powers, since its horns had the ability to glow.
