No, not the Warhammer-universe mercenaries, but the real thing, in real-world wars. The Library of Congress’ excellent Flickr account, where they share all sorts of treasures from their huge photo archives, put this image up earlier this year:
"The animal seeks for wounded men lost on the battle-field; he searches in holes, ruins, and excavations, and hunts over wooded places or coverts, where the wounded man might lie unnoticed by his comrades or the stretcher-bearer."
That lead to some Google searching and the discovery of several interesting articles about military dogs on the Western Front, primarily as ambulance dogs or messenger dogs. There’s another LoC image on Flickr that has some very good links in a comment just below it, a few of which I’ve reproduced here.
The fascinating Out of Battle blog has a pair of posts from 2008, one on messenger dogs and the other on ambulance and other uses of dogs, with some further links and resources. Over on the always-valuable Internet Archive there’s a 1920 book called British War Dogs, Their Training and Psychology, downloadable in the usual variety of formats, including PDF with the original illustrations and diagrams.
I’m not (yet!) into Western Front Great War gaming, but if you wanted a unique unit amongst your trenches, a dog and handler could be done quite easily with a spare infantry figure and a dog — quite a number of manufacturers make dogs in both 15mm & 28mm. From the look of the period photos, most of the dogs were collie or terrier types, not very large, which makes sense. The ambulance packs could be sculpted with a bit of milliput or greenstuff, and a messenger collar would be even easier to add.
Unleash the dogs of war!