There are several legends behind this statue, but the most famous is the one about Duke Godfrey III of Leuven. In 1142, the troops of this two-year-old lord were battling against the troops of the Berthouts, the lords of Grimbergen, in Ransbeke (now Neder-over-Heembeek). The troops put the infant lord in a basket and hung the basket in a tree to encourage them. From there, the boy urinated on the troops of the Berthouts, who eventually lost the battle. [...]
The statue is dressed in costume several times each week, according to a published schedule which is posted on the railings around the fountain. His wardrobe consists of several hundred different costumes, many of which may be viewed in a permanent exhibition inside the City Museum, located in the Grand Place, immediately opposite the Town Hall. The costumes are managed by the non-profit association The Friends of Manneken-Pis, who review hundreds of designs submitted each year, and select a small number to be produced and used." (Text from Wikipedia).