Kinoko Inu: Mushroom Pup (2010)

Blurb
A depressed author. A concerned editor. An eccentric mushroom researcher. And a strange pink mushroom … that turns into a dog?! Put this all together and you get Kinoko Inu.

Mushroom Pup tells a heart-warming tale of the power of friendship, a dog’s love, and the dangers of dwelling on regrets. With the help of his friends and the mysterious Mushroom Pup, Hotaru Yuyami, a grieving picture book author, slowly begins to move forward after the tragic loss of his beloved dog, Hanako. Full of laughs and warm moments, Mushroom Pup is sure to find a place in your heart.

Source: www.anisearch.com/manga/12602