The breed's trickiest relationship
Of all the questions about Chihuahuas, this one deserves the most honest answer. A Chihuahua's loyalty is usually aimed at people, and specifically at his people. Toward other dogs, the typical Chihuahua ranges from indifferent to openly bossy: he may guard his owner's lap, bark and lunge at large dogs on walks, and attempt to run a household containing dogs five times his size. That confidence is not courage so much as poor risk assessment, and it is dangerous, because a retaliating grab from a big dog can seriously injure or kill a dog who weighs only a few pounds.
Why so many Chihuahuas struggle with other dogs
Much of it is upbringing rather than blood. Chihuahua puppies are often under-socialized, carried past other dogs instead of learning to greet them, and their early warnings, staring, stiffening, growling, get ignored or laughed at because they come from something so small, so the dog learns to go straight to lunging and snapping. Fear does the rest: a dog who experiences the world from ankle height, surrounded by looming bodies, has reason to feel defensive. Genetics plays a part too, since the breed was not shaped for pack work or cooperative hunting, but a well-bred, well-socialized Chihuahua is usually neutral-to-friendly with dogs he knows.
Living with a resident dog
A Chihuahua can share a home happily with another dog when the match and the management are right. The best partners are calm, tolerant dogs, ideally not vastly larger, and never rough players who bowl smaller dogs over. Introduce them on neutral ground with parallel walking, feed in fully separate areas from day one because food guarding from the Chihuahua is common, and give each dog his own bed and resting space. Supervise play indefinitely: even friendly wrestling can injure a Chihuahua's legs or spine, and he will rarely opt out himself, so the opting out is your job. Separate them when you go out until the relationship is well proven, and many owners separate always.
Out in the world with other dogs
Walk your Chihuahua on a harness, keep distance from unknown dogs rather than letting them rush in to say hello, and never let him charge up to a strange large dog while barking, no matter how confident he seems. A retractable leash in a busy park is an accident looking for a place to happen. Reward calm looking at other dogs from a comfortable distance, cross the road when you need to, and pick him up only when a situation is genuinely unsafe, then put him down again once it passes, so being scooped up does not become his default answer to every dog he sees.
Managing two dogs without muddles
Households where a Chihuahua lives with another dog run on small separations, separate meals, separate beds, separate lead walks some days, and those routines fall apart when they live only in one person's head and that person is out. Writing the feeding spots, walk order, and supervision rules down for everyone removes the guesswork. In KinsLe Care Teams, you can assign each dog's feeding, walking, and care duties to the people who help, with completed tasks marked by a timestamp, keeping a fragile little dog's careful routine intact whoever is home.