The most famous "running of the bulls" are found in Spain and Mexico. Anyone can participate and show off their bravery (or stupidity) and run along the streets with these large animals. It is very common for someone to get trampled or gored during these events.
Now I like to run, I can enjoy a race, and I can even appreciate a little taste of danger to get the adrenalin going. But why one would put oneself in that situation on purpose is beyond me. I think sometimes those guys are just out for a jog minding their own business and next thing they know someone got the bulls running next to them and some fancy foot work is needed to get out of there. That's what happened to me anyway. I was biking along minding my own business when Devon decides to startle the beasts by yelling some Indian war-cry at them. They get into a full sprint right next to me with this crazy, confused, yet angry look in their eyes. And suddenly one turns and darts right at me. I peddled as fast as my chicken legs would carry me and had to swerve off the trail as this ten-ton monster just misses my rear tire!
So as far as races go, I don't recommend an encierro - you won't ever catch me on a road trying to outrun one of those...unless you're Mike and you're in Grover...then I suppose you already have.