Hi!
Jupiter's great Red spot is basically a storm like a hurricane, that has been raging for at least 300 years (which is how long we've been observing it). How exactly it formed is not known, but it may have formed spontaneously in Jupiter's turbulent atmosphere. Check out
http://www.npaci.edu/enVision/v14.1/marcus.html for more information about some computer simulations scientists have been doing to figure this out.
cheers,
Sarah