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Trusty is the neighbor of Lady, and possibly the oldest dog in the film. According to Jock and himself, he was once a service dog working alongside his grandfather, the esteemed Old Reliable, to track down and capture criminals. In one scene of the first film, Jock mentions to Lady of the "bygone days when he and his grandfather were tracking criminals through the swamps," indicating that he is from Louisiana. His Southern accent and way with manners further confirm his roots.
Eventually, Trusty's age eventually caught up to him, which is when he supposedly lost his sense of smell for a number of years. Though Trusty firmly believes his strong sense of smell remains intact, Jock believes otherwise (though he refuses to admit it, not wanting to hurt Trusty's feelings).
Now retired, Trusty spends his days alongside his best friends, Jock and Lady, who hold a strong kinship with the old dog.
Trusty can usually seem absentminded and a little clumsy, but is a loyal, sweet and respectful friend to Lady and everyone else. He has a large way with manners referring to Lady as "Miss Lady, ma'am." He can also be a blabbermouth, and regularly connects a story or topic to a piece of advice given to him from Old Reliable, with a statement going as followed: "As my grandpappy, Old Reliable used to say, uh... Don't recollect if I've ever mentioned Old Reliable before," to which his friends would reply by stating he has.
Trusty first appears in the beginning of the film where he is asleep, dreaming of his glory days of tracking down criminals in the swamps of Louisiana with his grandfather, Old Reliable. Jock mentions to Lady that poor Trusty has long lost his sense of smell. When Trusty finally awakens, he is delighted to see the new collar given to Lady and comments that she has grown up so fast.
Trusty later appears with Jock, as Lady is depressed at the fact that Jim Dear and Darling have not been giving the love and respect that she usually gains from them. They realize that Darling is having a baby and tries to explain with no luck until Jock does so. They briefly describe what a baby is and is interrupted by Tramp. Trusty tells Lady not to listen to Tramp's stories.
After Lady's heartwarming and heartbreaking adventure with Tramp, Jock and Trusty then appear as he visits her. Trusty is then seen feeling guilty at the fact that he misjudged Tramp after he saved Jim Jr. from a vicious rat. To redeem themselves, Jock and Trusty rush to find the dogcatcher's wagon, where Jock realizes that Trusty never lost his sense of smell; in fact, Trusty's sense of smell is so acute that the rain concealing the trail helped him track it down. Trusty and Jock successfully stop the wagon, but Trusty was caught in the wheel, leaving him seriously injured, seemingly killed. However, on Christmas Day, it was shown that he survived with a broken leg, and Trusty joins, Lady, Tramp and their family for Christmas.
Trusty reappears in the sequel and first appears in the beginning as he watches Lady, Tramp, and their family stroll through the neighborhood. He later appears after Scamp has run away. Annette, Collette, and Danielle bring the news and that's when Trusty brought back one of his old stories and mentioned the time he saved Tramp. Trusty joins the group as they search for Scamp. In the end, everyone is reunited once again.
Trusty's howling and his actions of doing so through the streets were given to Toby from The Great Mouse Detective. Also, his scene of almost scaring the dogcatcher was recycled in The Great Mouse Detective as well.
It's ironic that Trusty lost his sense of smell, as bloodhounds are arguably the dog breed with the best sense of smell.
Originally, Trusty was to be killed by the dogcatcher's wagon, but Peggy Lee cried when she saw it and begged Disney to avoid another traumatic scene like in Bambi, so Disney ordered the animators to make him just be injured on Christmas Day.