Throwflame is selling a robotic dog called the "Thermonator" for $9,400. You operate it over WiFi or Bluetooth with a remote control, and it can shoot flames up to 30 feet. The website says they're legal in all 50 states. You just need a special permit in California and Maryland.
They're not pushing the guard dog angle, but they do list eight different things it's perfect for. They include controlled burns - ice and snow removal - clearing weeds - pyrotechnics or entertainment - and also pest removal.