On March 12, 1961, a character named Dennis the Menace debuted twice, on either side of the Atlantic, mere hours apart, and by two people who had never met.
So, here's the story from the American perspective first: In March 1951, a cartoon called Dennis The Menace began, syndicated across 16 newspapers at first, about a little boy in Wichita, Kansas. Dennis Mitchell, the hero, is five and a half.
Also in March, and also in 1951, the Beano comic, a staple of British childhoods since 1938, began their new cartoon series, called Dennis the Menace. There is no possibility that either of the creators of either comic strip could have known that the other was being created.
The UK Dennis does not have a surname, nor does he accidentally find himself creating trouble wherever he goes. He does create the trouble, but it's entirely on purpose. He's the self-style wildest boy in the world. A real anti-hero. This Dennis is older than his namesake, and much, much tougher.