for Guinea Dog by Patrick Jennings
Rufus has been dreaming of getting a dog. His best friend has one. His worst friend has one. But his dad has a few objections: They whine. They gnaw. They bark. They scratch. They beg. They drool. Rufus pays no attention when his mom offers her think-outside-the-box suggestion, because she can't be serious. She can't be. She can be. And she actually comes home with a guinea pig. And if Rufus's dad thinks dogs are a problem, he won't know what hit him when he meets the Guinea Pig That Thinks She's a Dog. She barks. She bites. She'll eat your homework.
Publisher description retrieved from Google Books.