The answer for a German Shepherd in a multi-pet home
A German Shepherd should not eat food made for other kinds of pets, because his digestive system is one of the breed's known weak points and rich, unfamiliar food is the fastest way to upset it. Cat food leads the list of problems. It is formulated with more fat and denser protein than dog food, and a German Shepherd can eat several cat portions in one raid, so the load is both richer and larger than owners first assume. The common result is a day or two of loose stool, marked gas, and reduced appetite for his own balanced meals, which then tempts owners to offer extras and deepen the disruption.
Why the Shepherd stomach protests so loudly
A German Shepherd's long digestive tract and sensitive gut flora react to abrupt fat and protein changes more dramatically than many breeds, and stress or excitement around another pet's mealtime adds to the effect. Owners who are working to stabilise a Shepherd's digestion, perhaps after a run of soft stools, can lose weeks of progress to a single week of quiet cat-food raiding that nobody noticed. His intelligence compounds the issue: a German Shepherd learns exactly when the cat is fed, where the bag lives, and which cupboard door does not latch, so a solution based on telling him to leave it will fail long before his memory does.
Ranking the wrong foods a Shepherd might find
Wet cat food is the richest and the most likely to cause vomiting or diarrhoea in quantity. Dry cat kibble is slightly less rich per bite but easy to eat in large, fast mouthfuls, especially from an open bag. Food for ferrets is the most concentrated of all and should be stored like medicine, out of reach entirely. Pellets and hay-based foods for rabbits and guinea pigs mostly pass through a Shepherd as indigestible bulk, producing large stools and discomfort, while spilled bird seed adds oily calories. None of these feeds a Shepherd; each either upsets him, fills him with the wrong thing, or displaces the measured diet his digestion does best on.
Outsmarting a large dog who plans his raids
Store every pet food in hard, sealed containers inside latched cupboards, because a German Shepherd can open bags, flip loose lids, and operate some door handles. Feed the cat behind a closed door or in a room entered through a gate the Shepherd cannot clear, and pick up leftovers immediately rather than leaving food down. Teach a settled place behaviour for other pets' mealtimes, rewarding him for staying on his mat, but treat training as a backup to physical separation, not a replacement for it. His own meals should arrive on a predictable routine in his usual spot, since a Shepherd who trusts his schedule patrols less between meals.
When digestive upset after raiding needs a vet
Call your veterinarian if a German Shepherd who ate another pet's food has diarrhoea lasting beyond a day, vomits repeatedly, passes stool with blood, or becomes dull and uninterested in water. Large, deep-chested dogs who seem restless, drool, retch without bringing anything up, or develop a tight, swollen abdomen need urgent veterinary attention the same hour, not home observation. Bring a note of what he ate, the amount missing, and the timing, and mention any recent diet changes, because that context helps the clinic separate a simple dietary upset from something needing investigation. Keeping his real feeding record in KinsLe Care Teams, updated by whoever fed him, gives you that context without relying on memory.