Simple Q&A Pathfinder (By RAW) I

[Cieyrin]

A 386

You can't find it because (to the best of my knowledge) no such entry exists anywhere. The equipment table lists a guard dog and a riding dog, but only has descriptive text for the riding dog. Similarly, the Bestiary has stats for the riding dog, but not a guard dog.

My advice, which is decidely not RAW obviously, is to use the stats for the riding dog except remove the ability to use it as a mount. Alternately, you could also take the entry for a standard dog (the Small-sized one) and apply the "giant" simple template to make it Medium. More work, and may not produce different results (I'm too lazy to check it out).

A 386 continued Relevant post.

The basic take away is Guard Dogs are Dogs with the Guard purpose. Think of Pit Bull Terriers, who fit neatly within the 20 to 50 lb range talked about, which puts them in the same weight category as halflings and probably able to outwrestle your average one.