The short answer for a Bulldog eyeing another pet's bowl
A Bulldog should not be fed food made for other kinds of pets, and cat food is the mismatch that matters most in a multi-pet home. Cat food is formulated for an obligate carnivore, so it carries more fat and more concentrated protein than a Bulldog's own food. A Bulldog is a compact, heavy-boned dog with a famously delicate digestion and a strong tendency to gain weight, so that extra richness works against him twice: it upsets his stomach in the short term and pads his frame in the long term. Rabbit, guinea pig, and bird foods fail in the opposite direction, offering mostly plant material and seed with none of the balanced nutrition a Bulldog needs.
Why richness hits a Bulldog harder than most dogs
A Bulldog's gut reacts quickly to sudden fat, and owners know the signs: louder gas than usual, soft stool, and a dog who asks to go out more often through the night. His build adds a second concern. Extra weight settles on a Bulldog's chest and shoulders, making breathing harder work for a breed whose short muzzle already limits airflow, especially in warm rooms or after a short walk. Skin folds are the third place rich, off-plan food can show up, because a Bulldog prone to itchy, irritated folds does not need an uncontrolled diet change confusing the picture when you are trying to work out what he reacts to.
A stolen mouthful versus a habit
A Bulldog who grabs one mouthful of cat kibble while you are filling the bowl does not usually need a vet call. Watch him at home for vomiting that repeats, stool that stays loose into the next day, loss of appetite, or unusual quietness, and mention the episode if any of those appear. The picture changes when the raiding is daily. A Bulldog quietly finishing the cat's leftovers every evening is eating a second, richer dinner that no one has measured, and his own measured meals then get blamed unfairly when his weight climbs. Ferret food is even more concentrated than cat food, so treat a ferret's stash with the same seriousness.
Setting up a home where the Bulldog cannot graze
Feed the cat somewhere the Bulldog's wide, low body genuinely cannot follow: behind a gate he cannot push through, on a raised surface he cannot climb to, or in a room with the door closed during meals. Pick up any uneaten cat food instead of leaving it down for grazing, because a Bulldog will patrol for it long after his own bowl is empty. Feed small pets such as rabbits inside their enclosure and keep their pellets and hay in a latched bin, since a Bulldog investigating an open bag can eat a surprising amount of dry plant food and end up constipated and uncomfortable.
When to call the veterinarian about what he ate
Contact your veterinarian promptly if a Bulldog who raided another pet's food vomits repeatedly, develops watery stool that does not settle, strains, drools heavily, or seems painful around the belly. Call sooner if he ate a large amount at once, ate food meant for a species whose diet includes ingredients dogs tolerate poorly, or if he is a puppy or an older Bulldog with an existing health condition. Bring the other food's packaging or a photo of its ingredient panel, because knowing exactly what he ate helps the clinic judge the fat load and anything else of concern far faster than a guess. Writing down who fed which pet, and noting any raids with a timestamp, is easier when KinsLe Care Teams holds those feeding duties for the household in one shared place.