When you complete the quest this poor hungry puppy offers you, he joins you on your Tillers farm! He sits beside Farmer Yoon and sometimes walks a few rounds through a pool on your farm. Don't think there is anything but a cosmetic value, but still very fun.
With the rep buff from Hallow's End, I received 600 rep points with The Tillers upon turning in the quest. So it's very much worth doing, in my opinion. :)
As of 5.2 with the Grand Commendation system, you will gain double the amount of reputation. Now you'll really want to look for the dog.
Must be at a certain point in Revered with The Tillers before this quest will open up to you. (For me, I didn't see a yellow ! mark until I was past half-way through Revered.)