Why is it there?
Well that's what you call an ambiguous detail. You see, in short stories or comics, there isn't always time to develop or explain every aspect of the story; therefore some stories prefer to focus on the more interesting parts while leaving the minor details up to the imagination of the reader.
So for example we could say in this case, this werepuppy was left by someone in a desperate hurry and with little choice (perhaps in battle with the forces of darkness) and when the girl picked it up, the puppy instinctively bit her to make her let go. The pup wasn't fully aware of what's going on because he didn't seem know that lycanthropy can be passed through bites.
But if you don't think the same, well that's up to you to decide! Those details ambiguous so that you can have fun using your imagination.