This was a new one on me. I guess we still used a lot of horses at the time. I assume we didn't use many dogs back then.
It very well could be for dogs too. Dogs were used extensively during the great war for multiple purposes. The germans in particular used them with great success in searching out breaks in the wire, but mainly for relaying messages between units more reliably than pigeons. They would often also have light packs that would carry ammunition and hand grenades to pinned down German "storm trooper" units on the assault. I would expect that the U.S. and allies would also have used dogs for similar purposes. http://www.whale.to/b/germansoldierdog.jpg
Here's a picture of an assault trooper posing with two dogs. there's another photo in a book I have called "WWI trench warfare" which shows three German assault troops pinned down in a shell hole in no man's land pulling in a dog carrying ammunition, hand grenades, and packs of cigarettes of all things. Anyway, I don't mean to hijack the forum: just always found the first would war to be fascinating and wanted to share.