I did a quick search for the word 'dog' and nothing came up, but I can't believe that no one will have pitched this. I'll pitch it anyway, just in case.
I was thinking that a dog would be a useful ally for a hearthling. He could help to tame animals, protect (maybe at his own peril) his hearthling owner should a boar or a bear attack. He could sniff out wildlife, raid ant nests, guard your house while you're away. He may occasionally dig up the odd pile of dirt in your garden or chew on your furniture but his heart is in the right place.
As for how you would acquire a dog, I'd say they're tame wolves. Wolves would be very strong and travel in packs, and as such only highly experienced hearthlings would be able to tame them.
What do you think?