from wiki:
The 24m inch tall bronze statue on the corner of Rue de l'Etuve and Rue des Grands Carmes was made in 1619 by Brussels sculptor Hieronimus Duquesnoy the Elder, father of the more famous François Duquesnoy. The figure has been repeatedly stolen: the current statue dates from 1965. The original restored version is kept at the Maison du Roi/Broodhuis on the Grand Place.
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.
Another legend states that in the 14th century, Brussels was under siege by a foreign power. The city had held its ground for some time, so the attackers conceived of a plan to place explosive charges at the city walls. A little boy named Julianske happened to be spying on them as they were preparing. He urinated on the burning fuse and thus saved the city. There was at the time (middle of the 15th century, perhaps as early as 1388) a similar statue made of stone. The statue was stolen several times.
Another story (told often to tourists) tells of a wealthy merchant who, during a visit to the city with his family, had his beloved young son go missing. The merchant hastily formed a search party that scoured all corners of the city until the boy was found happily urinating in a small garden. The merchant, as a gift of gratitude to the locals who helped out during the search, had the fountain built.
Another legend was that a small boy went missing from his mother when shopping in the centre of the city. The woman, panic-stricken by the loss of her child, called upon everyone she came across, including the mayor of the city. A city-wide search began and when at last the child was found, he was urinating on the corner of a small street. The story was passed down over time and the statue erected as a tribute to the well-known legend.
Another legend tells of the young boy who was awoken by a fire and was able to put out the fire with his urine, in the end this helped stop the king's castle from burning down.