Guess one of the few things I have to say is great about my parents is that my mom's a major animal person. We've always had a dog in the house. But my parents never offered incentives to do well like that. My incentive was if they didn't like my grades they'd come down like a ton of bricks. If everything was fine, nothing really happened except for special occasions like in 11th grade when I was accepted into NHS. Aside from things like that, no incentives. Just a whole lot of grief.
But good luck getting your dog. Depending on your mom, that might be an easier sell than rats... My mom loves animals, but if I ever mentioned a rodent as a pet... Oh, boy! She can't stand them. At least from what I've seen, even though rats take up less space and can easily be avoided by family members, people seem much more adverse to having them in the house than a dog or a cat.