Dennis appears 62 times at rank 1644 on male pets. Like Brian and Gary, Dennis is a resolutely normal human name that functions as a pet name through sheer incongruity. And unlike those names, it has a specific mischievous streak built in via its most famous cultural bearer.
The Dennis the Menace Factor
Dennis the Menace has existed in two parallel versions since 1951: the American version (a cheerful, accident-prone five-year-old) and the British version (a genuinely chaotic boy who makes his neighbors' lives difficult). Either version supplies a useful brief for a dog who consistently finds trouble despite the best intentions of everyone involved. The name is an invitation to misbehavior that owners extend knowingly.
The Deadpan Human-Name Appeal
Dennis joins Brian and Lewis in the category of pets with names so human that the absurdity is the point. It works best on dogs with enthusiasm and questionable judgment: Beagles, Jack Russell Terriers, and any breed that gets into things it shouldn't. See the human-name context at /names/dennis.
The Counter-Reading
Dennis is a name that requires the dog to earn it. A perfectly behaved, obedient dog named Dennis is a mild joke. A dog who deserves the name will make it feel inevitable within the first week. Either way, it's worth choosing for the right dog.