Buck is Sheriff Sam Brown's impetuous horse who is always hungry for action and adventure in the town. He is best friends with Rusty, the sheriff's dog, and they both seem to enjoy playing tick-tack-toe together. Buck has a tendency to act immaturely at times, but he can also be very helpful when he wants to. Being a stallion, Buck usually has a mind of his own.
Buck is first seen with his owner Sheriff Sam Brown hammering a wanted poster of Alameda Slim. Later, Sam rides on Buck to give Pearl Gesner a bank notice, explaining that the banks are going to auction off her home if she can't pay the full amount she owes them, because they are losing so much money due to cattle rustlers leaving behind so many bankrupt ranchers. When Mrs. Calloway asks if he is proud of reporting this horrid news, Buck arrogantly answers, "Yes." When Sam suggests to Pearl to sell off a few of her animals to pay the debt, she chases him off her property in a fury, sending him and Buck running.
Buck is later seen having a daydream of fighting bandits. Buck uses his reins to knock the guns out of the bandits' hands, then defeats them all with his martial art skills. After gloating by making a few victory poses, Buck is woken up from his daydream by Rusty's voice. Rusty then reminds Buck that it's his move at tick-tack-toe, which they were seen playing together. Buck becomes distraught, knowing that he will never get a chance to fight bandits, but Rusty tries to reassure him that there are other things he can do besides be 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. When Rico tells Sam that he needs a fresh horse to help him capture outlaw Alameda Slim, Buck immaturely shows off behind them and Rusty. Rico eventually selects Buck, much to the horse's delight, and then saddles him up. Buck exclaims happily that he's wearing Rico's saddle, and the two ride off together. At a cattle drive, Buck encounters the farm cows who had the same idea, and gets into a brief argument before they leave him to wallow in his self-pity when Rico chooses another horse to assist him on his "mission" because from the way he saw Buck's exchange with the cows, he interpreted it as Buck being "skittish around cows." He then tries to enlist some ranchers to return Buck, only for the stallion to escape the ranchers and run off to prove himself to Rico again.
Buck later decides to travel to Echo Mine alone and capture Alameda Slim himself. Upon arriving there, Buck comes across Junior the Buffalo who refuses to let the horse pass, and says that the only critters that get by him are cows. Maggie, Mrs. Calloway and Grace then show up along with Lucky Jack (a jackrabbit who helped the cows reach Echo Mine), and Junior lets them all pass. When Buck questions Junior about this, the buffalo states, "They're cows," and when Buck brings up the rabbit, Junior replies, "Well, obviously, he was with the cows." Buck tries convincing Junior that he was with the cows too, but to no avail.
Sometime later, Buck manages to trick the same horse from before into leaving his post so he could get back with Rico. However, when Buck gloats, he is overheard by Junior who snorts angrily, as he had been right behind him. Junior is later seen chasing Buck around Echo Mine until he is saddled up by Rico, to which Buck happily gallops off with him. The chase eventually ends when all groups collide with the train.
When Slim reveals that Rico is actually a mercenary who was working for the outlaw all along, Buck is left feeling betrayed and heartbroken. Seeing the cows being forced into the train by Slim's goons, the horse turns on his traitorous rider by flailing about wildly and manages to buck Rico off his saddle. When Rico attempts to shoot Buck afterwards, Mrs. Calloway and Maggie manage to take out the fake bounty hunter. Buck then switches sides by accompanying the cows and helps defeat the Willie Brothers and Mr. Wesley. He also frees all of the stolen cattle that Slim and the Willies had been harboring in Echo Mine.
Later, Buck, Lucky Jack and the cows hijack a train to get home, but it gets in the path of another oncoming train. When the switch that reverses the train track breaks, Buck, thinking fast, wedges Lucky Jack's wooden peg leg in the switch's opening and pushes him down like a lever, thereby saving the cows from a possible collision while the driver of the oncoming train yells, "Roadhogs!" Lucky Jack tries to pull his peg leg out of the lever while he and Buck wish the cows good luck, and the two eventually arrive at Little Patch of Heaven after Slim is arrested when his disguise is removed.
Buck is last seen teaching the piggies karate and dancing with all the other farm animals as they celebrate Pearl's farm being saved and re-opened.
- Buck is very similar to Chance from the Homeward Bound films in that they are both impetuous and act immaturely.
- When Buck says, "Got milk?" to the cows, his dialogue is based on various TV commercials featuring the humorous tagline: "Got milk?"