There is a Dutch name Hade which has the alternate forms Hatte and Hatten, and goes back to a Germanic word meaning "battle, strife, quarrel." At the end of its entry on Hade, my Dutch name dictionary (
Woordenboek van Voornamen by J.
Van Der Schaar) adds the following sentence:
Hierbij ook de aangetroffen vorm Hattum? I can't find a meaning for the word "aangetroffen", but as near as I can figure out
Van Der Schaar is saying that maybe the odd form Hattum is another variation of Hade. This is obviously an obscure name even in the Netherlands, as
Van Der Schaar isn't quite sure what to make of it. :)