What does the game ending mean？

[Emerald♛]

why the cat live with the dog and the mouse？

[JohnnyBleu]

Barkley stopped his master, Otto, from doing terrible things by killing him in Gary's body. Gary's consciousness was transferred into the rat's body. Kiki is still Kiki. They all live happily ever after. Also, if you saw the "Anna" message in one of the secret rooms in the game, she is Gary's deceased spouse and Kiki remembers the fond memory from the recording. Essentially, this game sees animals surviving humans. Hope all that info helps explain the answer you're looking for.