Our two current pets were not always friends. As noted in this blog, kitty came to us from a home that had dogs, but those were 'her' dogs and once she came to our home, the dogs here were not her dogs, which made her skeptical of them.
To their credit, the three dogs were more than a little curious about kitty, back then. Yes, they'd had a kitty in the house, but Pandora was old, she was settled in her ways, and she mostly ignored the dogs. Therefore, they never got to know here, or to understand that she was just another creature, much the same as they.
Our dogs, at the time, were curious beyond compare about all animals. If you brought the animal into the house, their curiosity was multiplied exponentially. I do not think, however, they understood that other animals were the equivalent of them. In other words, other animals ('other' being the operative word here, as in anything that wasn't a dog - including a visiting ferret), were alien beings likely from another planet. We may argue that dogs don't understand concept such as 'planets' but the idea is what is important here. To Chester, Olive, Emily and the visiting granddogs, Twiggy and Onyx, cats or ferrets - and one would assume guinea pigs or other kinds of pets, heaven forbid we should bring a bird into the house! - were creatures of unknown origin, and certainly not 'dog'.
Which is true. They aren't "dog."
Over time, the pets in the house changed. The dogs came to understand that the kitty, Molly, was staying. That she was a member of the household. That she was maybe not the equivalent of each of them (after all, they are so unique and special, no other creature could match their awesomeness), but she was close to their equivalent. She was special in some other way. They came to know this and accept it, although for a good bit of time, it was clear their curiosity was not satiated no matter how close she allowed them to come to her, and as such, they viewed her as a visitor that wasn't leaving.