As befitting a long-running, best-selling book series, Hank the Cowdog has many popular supporting and guest characters.
- Slim's Tomboy with a Girly Streak, dog-loving girlfriend Viola is a frequent favorite among the human characters despite only being a secondary character.
- Coyote White Sheep Missy doesn't appear often, but her sweet nature and Star-Crossed Lovers interactions with Hank make her character one that fans are always happy to see show up.
- Sinister the Bobcat's appearances so far can easily be counted on one hand and he is The Voiceless until Book 80, but his intimidating visage, the humorous way Hank ends up Mugging the Monster while trying to pounce on Sinister in the bobcat's first scene, and the Affably Evil attitude Sinister displays after finally deigning to talk to Hank all make him a beloved Rogues Gallery member.
- Cloud Cuckoo Lander witch owl Madame Moonshine and her rattlesnake bodyguard Timothy tend to add a lot of enjoyable color to their rare appearances.
- Bruno the Boxer only appears for one chapter in the first book as someone for Hank to taunt, but his name is well-known to the fanbase due to how frequently read and quoted that scene is.
- Little Alfred's cousins Amy and Ashley only have a couple of appearances but are fondly remembered for their cute Pet Dress-Up interactions with Hank and Drover.
- Bub the monkey, the trained accomplice of a local burglar, is only in one story but is notable for being one of Hank's most wily and hilarious adversaries.
- Feed-stealing raccoon Eddy appears infrequently but is quite popular for his Friendly Enemy interactions with Hank and frequently successful Batman Gambits.
- The Mama Bear cat and her adorable kittens from The Case of the Haystack Kitties are in many fans' hearts for causing Hank to make an exception for his Animal Jingoism.
- Good-natured bumbler Happy the Labrador only has a few appearances, but the fans like him just as much as most characters.
- Frankie, the eponymous character in The Case of the Fiddle-Playing Fox, is hard to forget, with his gentlemanly Cunning Like a Fox behavior and methods of getting food.
- Gentle Giant Sleepyhead Brewster only appears in The Case of the Midnight Rustler, but is still one of the best-liked dogs in the books for some fans.
