Beulah

Beulah is a collie who resides on a local ranch, and with whom Hank is madly in love. He is constantly trying to impress her, which usually results in his making a fool of himself instead. Beulah lives on her ranch with her boyfriend Plato, a bird dog who Hank despises. Although she likes Hank as a friend, she does not return his love, and tries to let him down easy for fear of hurting his feelings. Despite this, Hank continues to be convinced Beulah is in love with him, and would express it if it weren't for Plato.

In the 26th book, The Case of the Kidnapped Collie,  she is kidnapped by  Scraunch, and Hank has to save her.