After a lot of research, we were able to come up with some legends, which may be the basis of Manneken-Pis' story. Oh, FYI, his name means little peeing man in Dutch.
Quite Harry Potter-ish is the first legend. It tells the story of an old witch who lived in Rue de l'Etuve and who convicted a little boy of a sad punishment. He had relieved himself on her front door, so she decided to transform him into a stone statue. Fortunately, a good old man appeared with a statue similar to the little boy. When the witch was ready to put a curse on the little boy, the old man swapped the boy for the statue.
The second one is also about an illegal wee-wee... "Little Julien" was knee-high to a grasshopper when he went pee pee against the door of a saint hermit. The old man heard the weird noise and came out of his house. Then he saw the little boy, transformed him into a statue, and condemned him to keep doing the act... forever... However, the end of the legend, which was too scary for the kids, was changed. The father ordered a statue, similar to his son, and the day it was presented, the little boy came back to life.
The third one tells an epic story, when Brussels was besieged and had been resisting attacks for several days. Besiegers set up fire to burn the city before they left. Fortunately, a little boy who was walking down rue de l'Etuve saw the burning wick. He didn't know where to find water, so he started to pee to put it out. The Bourgeois heard about it and erected a statue honoring his epic act.
The fourth one is mischievous and goes back to the Crusades. The count from Hove lived in Brussels with his wife and son Godefroid. The count steadily hosted his glorious combatants. Once, he asked his 5 year old son, a resourceful kid, to walk in front of the troops for welcoming the combatants. But the resourceful child spent his time watering the procession. Then a statue was erected by the count and countess to make up for the insult inflected upon the combatants.
The fifth tells the story of a Bourgeois child who lost himself in the crowd. His father searched frantically for him in the Brussels streets... After five days, he saw his son relieving himself at the corner of rue de l'Etuve and rue du Chêne streets. The Bourgeois was so happy and decided to erect a statue to immortalize the moment.
The sixth goes back to the 8th century. A lord's wife gave birth to a baby who peed so high that he spattered the beard of Vindicien, an Arras bishop. The kid was named Manneken-Pis. Soon after, Vindicien passed away. There was a problem: where to organize the baptism and who would officiate it? A woman called Gudule would bless this baby. But the lord, seduced by the woman, left his house and went to Gudule's. She welcomed him, ignorant of his bad intentions. Gudule was shocked and, to punish him, she declared: "Your son is going to stop growing and will never be able to stop peeing."
The last and most plausible legend
The most plausible one is based on historical facts. In 1142, Godefroid III, duke from Lotharingie, was born. Unfortunately, his father, Godefroid II, passed away soon after his birth. Two vassals, Gauthier Berthout and his brother Gérard de Grimbergen, took advantage of the situation to fight over their new born liege lord. The Gaasbeek sire asked for the presence of the young duke on the battlefield. Then the crib was hung from a oak tree's branch, in Ransbeek. The baby's army was four times driven back, at this point they seemed to have lost the fight... But they were suddenly galvanized, watching the baby peacefully doing the famous act! To commemorate the victory, a fountain was erected in Brussels, and it was called Manneken-Pis. Moreover, the oak tree was uprooted and replanted at the Rue du Chêne (meaning oak in French).
Mannenken-Pis and family?
Even if Manneken-pis is an original, he has some 'brothers', in Grammont, in Kobe, in Spain, in Osaka, in Colmar, and he also has a sister: Jeanneke-Pis, located in Brussels, in the impasse de la Fidélité. Yep, Brussels has a lot of peeing statues...