This quote comes from the 1977 Monster Manual:

Dog, War: These are simply large dogs which are trained to fight. They are loyal to their masters and ferocious in attack. They are typically protected by light studded leather armor and a spiked collar. The number appearing depends on their masters.

In this entry, war dogs have an AC of 6, 2 + 2 HD, and do 2d4 damage (that’s right, better than an AD&D broad sword). In fact, they are pretty badass. And as far as I can tell, there is nothing anywhere in the rules proper prior to second edition suggesting that dogs are standard dungeoneering animals. This first Monster Manual entry is pretty clearly intended as an opponent, down to the “number appearing” language. (Incidentally, I love that bit about the spiked collar. Every war dog should have one.)

They are not in the 3 LBBs. They are not in Holmes. They are not in Moldvay Basic or Cook/Marsh Expert. I’m not all that familiar with BECMI, but it doesn’t look like dogs show up in the Rules Cyclopedia, so I’m guessing they are not in Mentzer either. A pair of war dogs is one of the items that can be found in a Robe of Useful Items (AD&D DMG page 153). Some dogs do show up in the 1978 PHB on page 36 under livestock:

Dog, guard 25 g.p.
Dog, hunting 17 g.p.

Now, a guard dog or hunting dog seems like a far cry from armored dungeoneering war dog to me, but I can see how a player might see that entry in the PHB and then say “cool, what are the stats on those?” and come upon the war dog entry in the Monster Manual (which, arguably, is the closest fit superficially).

War dogs don’t show up in the equipment lists until second edition PHB (which also contains rules for training animals in the chapter on proficiencies). Second edition prices:

Guard 25 gp
Hunting 17 gp
War 20 gp

So much for the official TSR rules. I am sort of fascinated with when this trope developed, but really it is neither here nor there. What if we don’t care too much about the old rules and just want some guidance for using dogs?

More recently, Daniel Proctor (of Labyrinth Lord) wrote a special supplement, Dogs in the Dungeon (discussed in this forum thread). A post in that same thread alerted me to the fact that this topic was treated twice in Dragon, once in issue 103 (pages 26 – 28, for first edition) and once in issue 237 (pages 18 – 22, for second edition). From that article in issue 103 (in 1985):

Dogs can be useful allies or formidable opponents, depending respectively on the inventiveness of the players and the intelligence and imagination of the DM; with their keen senses, even the smallest dogs are the bane of thieves — PCs and NPCs alike — and a war dog is more than equal to the average hired swordsman or first-level fighter. Many other uses will doubtless suggest themselves to the thoughtful referee or player. Dogs may be employed as scouts, guards, or hunters, and are almost as useful and much less demanding than hirelings or henchmen.

So at least by that time, using dogs essentially as retainers seems to have been a common idea. And from the article in issue 237:

This article provides a method for designing canine NPCs. Included are numerous skills and gaming suggestions to turn man’s best friend into a furry adventuring companion.

The article goes on to (I kid you not) define a specialized set of canine ability scores (intelligence, aggressiveness, strength) and a skill system for dog training (some example skills: mounted heel, resist instinct, and stay). I find the complexity of some of these systems somewhat baffling. Did anyone ever really think it would be a good idea to have a specialized skill system for dogs?

Molossian Hound (personal photo, from the British Museum)