ANSWERED BY MOONBEAM
First of all, who has a pet rat? That’s like having a pet mosquito or a pet bubonic plague.
Second, your theory is a myth. All animals can’t get along. Some are just too tasty. Some are too dumb. Just start talking to a rabbit about politics or religion. I’m telling you, all bets are off.
Your guys are being nice to each other because they want you to be happy. But no one’s having a good time. Look at them. Your dog’s tail is between his legs. Your rat looks like he’s going to lunch with a boa constrictor. And your cat is trying to decide who to attack first. Luckily cats are indecisive and easily overwhelmed.
My advice is to lock the cat and the rat and a few of the rat’s friends together and let them discuss the meaning of life. They’ll enjoy that. Meanwhile, take the dog for a nice long walk and give him plenty of treats.