Someone speculated that the cat was protecting the pet rat but I don’t see that. I see a rat being friends with a cat and a dog wanting to play with the cat. The rat is just a bystander. He’s in the way. It is quite an interesting threesome nonetheless.
The same rules apply: these three have been raised together and they totally accept each other. There is no issue of one attacking the other. They are buddies. In other words they have been socialised to each other. There is a fourth player: humans. These three have also be socialised to humans. Pretty neat. It is funny that some humans are not socialised to other humans! The animosity between some groups of humans is shocking.