These werewolves exist only in the song. And they don’t seem to exist in te Netherlands either.
Translated from dichtbij.nl in the Netherlands:
‘Werewolf’ in Zeeheldenbuurt probably fantasy
Nicolai Brannan, editor
24 January 2014
Leiden – The story of, maybe, a werewolf in the Zeeheldenbuurt neighbourhood in Leiden is probably made up. Police have said this to the Leidsch Dagblad newspaper. Earlier this week there was a report to the police that a person dressed as a werewolf scared children.
The police assumes that the story was invented by a child with an active imagination.