The Dog is an animal and main antagonist in Chapter 1. The Dog appears in Over the Garden Wall.
The dog was originally owned by Beatrice's family as shown in the intro. After the family was turned into blue birds, they soon left their home and consequently their dog as well.
In The Old Grist Mill, the dog was first seen in the intro with a girl dressed in blue. The dog has arrived back to Beatrice's house and watermill which is currently inhabited by the Woodsman and is lured by the candy Greg constantly drops in order to make a candy trail back into the forest. When the dog arrives, Wirt and Greg mistake it for the beast the Woodsman had told them of and flee. They find temporary safety upon the roof of the mill but the dog has caught up with them. As a last resort, Greg takes the last piece of candy stuck to Wirt's cloak and tosses it into the river. The dog leaps after it only to get caught in the waterwheel, destroying the mill in the process. After coughing up a black turtle it had swallowed, the dog is revealed to be a normal animal.
The Woodsman laments over his mill and reveals that the dog was not the Beast at all. Greg attemps to befriend the animal but he sets off and isn't seen until the end of the series where he has returned to Beatrice's home and family.