Rusty is an aging dog (possibly a basset hound) who watches over the town when his owner Sheriff Sam Brown is away. He is shown to be very serious-minded and has little tolerance over Buck's immaturity, although the two are best pals and work together quite well. They also have a thing for tic-tac-toe.
Rusty is first seen trying to snap Buck out of one of his daydreams by whistling, and reminds him that it's his move at tic-tac-toe, which they were seen playing together. Buck becomes distraught, complaining that he will never get a chance to fight bandits, but Rusty tries to reassure him that there are other things in life besides being a hero. However, Buck still thinks otherwise and then embarrasses Rusty with his foolish karate moves.
Sometime later, Buck is distracted by Rico, a supposed bounty hunter and the horse's idol, whom Buck claims to be one of the best, although Rusty shows little interest. When Rico tells Sam that he needs a fresh horse to help him capture outlaw Alameda Slim, Buck begins immaturely showing off behind them and Rusty, who smacks his face with his paw in shame. Rico eventually selects Buck, much to the horse's delight, and saddles him up. Buck exclaims happily that he's wearing Rico's saddle, to which Rusty replies apathetically, "Terrific."
Later, when Pearl and her livestock face losing their farm due to a bank notice that she hasn't been able to pay yet, Maggie, Mrs. Calloway and Grace take Alameda Slim's wanted poster and seek Rusty and Buck for help. Rusty sniffs Slim's wanted poster a few times and states that they will need $750 in reward money to save Little Patch of Heaven. The three cows then get into a brief mud-wrestling fight while Rusty watches in amusement.
Rusty is later seen looking down at their tic-tac-toe game in sadness while it rains, as he was lonesome for Buck after learning that the stallion had escaped some ranchers who tried to return Buck when Rico ditched him for another horse. Rusty then wonders if he will ever see his stallion friend again.
Rusty is last seen overjoyed at Buck's return. He then gets caught inside a rolling barrel that Jeb and Lucky Jack overturned while fighting and bickering with each other. The barrel eventually falls into the water trough, with all three of them laughing at the end.