Our dog is fat, lazy, and needs to lose weight. This is common in dogs her age, so we have a lower calorie dog food.
Our ferret is crazy, everywhere, and needs to gain weight. This is common in ferrets, so we have ferret food out all the time in addition to always trying to feed him treats (eggs, meats).
Big problem in our house is that they like each other's food. It makes sense why the dog likes the ferret food. It's high fat, high calorie, probably delicious. But I have no idea why the ferret likes the dog food. And our dog, being a sweetheart herd breed, just looks sadly at us while the ferret crunches down on her food. Sometimes we hear a bark but the ferret generally doesn't care. It's bad. If the dog eats ferret food it's usually a tiny bowl and the only downside is that the ferret can't eat it. But if the ferret fills up on low-calorie, filling dog food, he won't gain weight.
Today the ferret had food in his cage, upper level so the dog can't get to it, but still ran out as soon as he could to go investigate what was in the dog bowl. NOTHING, it turns out. So I took his bowl of ferret food over to the kitchen. Nope, he wouldn't eat it. So I dumped the ferret food in the dog bowl. Watson went to town. Ate a fantastic meal.
So maybe he just needs a giant bowl. Maybe he feels belittled by his little food dishes. in his head, he's a doberman.