Guru Guru Pon-chan(1997)

Ponta is a Labrador retriever puppy, the Koizumi family's pet. She's full of energy and usually up to some kind of mischief. But when Grandpa Koizumi, an amateur inventor, creates the Guru Guru Bone, Ponta's curiosity causes trouble. She nibbles the bone...and turns into a human girl!

Surprised but undaunted, Ponta ventures out of the house and meets Mirai Iwaki, the most popular boy at school. When Mirai saves her from a speeding car, Ponta changes back into her puppy self. Yet much has changed for Ponta during her short adventure as a human. Her heart races and her face flushes when she thinks of Mirai now. She's in love! Using the power of the Guru Guru Bone, Ponta switches back and forth from dog to girl...but can she win Mirai's affections?

(Source: Del Rey)

Note: Winner of the Kodansha Manga Award for Shoujo 2000.