"Perry the Platypus! How did you escape my net? It was the mouse wasn't it?" — Dr. Doofenshmirtz asking Perry if the mouse freed him.[source]
An unnamed mouse appeared in The Fast and the Phineas and later in I, Brobot. In "The Fast and the Phineas" Perry tried to escape from the glass he was in by launching stinky cheese into Doofenshmirtz's pants which the mouse crawls into them, making Doofenshmirtz scream and the glass break. In "I, Brobot", Perry escaped from the net by spraying cheese on the rope. The mouse comes out of its hole and chomps on the rope, which tastes like cheese.
The mouse has white skin, pink eyes, white ears with pink insides, a red nose and a long pink tail.