He was a real historical figure, known for his satirical novel Simplicissimus (published in 1669). While the novel is a fictionalized account of his life, the story of him breeding a vicious dog to protect him is likely based on his real experiences as a tax collector, which was a dangerous job in those times.
It's important to note that the story of the dog is not historically documented, but it's a popular anecdote associated with Grimmelshausen.