Phew, this backstory is going to be a little long. Get comfortable. So both Rainy and Diesel volunteered to be part of an experiment. The goal was to find a way to regrow missing limbs, heal an otherwise permanent ailment (blindness, hearing loss, severe brain damage) things like that. The scientists would inject the dna of another creature that naturally has regenerating properties to the test subject and monitor how they would respond. Rainy (originally cat) was injected with only one dose of dna. Diesel (originally canine) was unintentionally injected with many doses of many different types of dna. The result was a mismatched jumble of all kinds of animals. Due to the multiple doses and odd conditions , his memories mixed up with the memories of the original holders of the dna, some worse then others. This made it very difficult to continue living the way he was used to. He was held at the experiment site until Rainy took him home with her, where they both live.