This spirit, one of the seven Amesha Spentas, was the personification of wisdom. The name literally means "good sense." Bulls, cows, oxen, and other animals were used to represent Vohu Manah on earth. It was a fairly important part of the mythology, if statuary counts for anything. The University of Chicago Oriental Institute has in its gallery a colossal stone bull's head (which you can see at http://www.oi.uchicago.edu/OI/MUS/HIGH/OIM_A24065_72dpi.html, although it doesn't give you a very good idea of the real size of this thing) taken from the entrance to the king's throne room at Persepolis, as well as other similar but smaller statuary from the same site.
In the Zoroastrian belief system, Vohu Manah's chief duty was wrestling his foe, the Daeva Aka Manah, whose name meant "bad sense" or "evil thoughts," for control over the hearts of men. This battle apparently was thought to take place inside the hearts of every individual human constantly, but Vohu Manah still had time for other duties. He was not only the good influence on people's hearts, but also the protector of the animal world and the guide to paradise. Being one of the Amesha Spentas and therefore just a step removed from Ahura Mazda, though, it's natural that he'd have a lot of responsibilities.
Later in the tradition, Vohu Manah assumed a special place as the son of the creator, Ahura Mazda. He was also fated to finally overcome his lifelong opponent Aka Manah on the day of the last conflict between good and evil. This could be a corruption in hindsight from Christian thought, or it could easily be the other way around. I'm not the one to judge.