Of my four cats, I am pretty sure (given the obvious fact that I’m attempting to apply aspects of a fictional universe to real felines) that Nikki would be a Slytherin, Cora would be a Ravenclaw, and Brodie would be a Hufflepuff.
Shadow, however, is harder to place. He is Gryffindor-ish in many respects. He is often a Fred-and-George-esque prankster in his interactions with the other cats. He also has this…”zest for life” thing going on that seems quite Gryffindor (as in, he gets Really Into whatever he is into. He is noisy about everything from eating to playing to walking/running. Etc.). He is stubborn, a natural hunter (he is the only one of the crew here to have caught and dispatched a full-grown rat!), and I dare say a bit of a show-off.
BUT…despite all that? I could almost see him ending up in Hufflepuff with his brother.
For one thing, he and Brodie are total BFFs in addition to being brothers (their interactions were even described as being suggestive of “bromance” by one person who encountered them).
Shadow may be far less subtle than his brother in engaging with the world, but he is so unremittingly loyal to Brodie (as well as to his home, and to what I can grok of his cat-code-of-ethics) that it might be this trait ends up taking precedence over the others). Which is quintessential Hufflepuff. But so much of him is also deeply Gryffindor, I rather suspect that he himself might have a tough time choosing between Houses if he has the opportunity.