The entry for dog on page 271 in the MM states that the dog is a small animal 20-50 lbs, and suggests also using the entry for coyotes, jackals, and African wild dogs. It seems doubtful that the game designers had a dog of that size in mind as the typical "guard" dog. The Riding Dog entry cites as examples collies, huskies, and St. Bernards. That being said, the logic of the light horse/light war horse proffered by Firelance appears to make sense. A small description of the guard dog in the PHB would have been nice. Oh, well.