well the scaredy rats in my real mario serie are one of the mutated animal specie.they are bigger and way scarier then normal rats.they are mostly seen in my second game in the last chapter(Luigis mansion) o yeah the game ends with the famous luigis mansion chapter ^^ but you do see them as well in the haunted house in the first game but sins luigis mansion is one year after you defeated bowser there more ghost and mutated specie have been left to grow stronger and scarier.those rats naturely leave you alone but if you are weaken in health and thet are in group now there see you as more easy prey.and if you treaten them they will strike naturely.there mostly just trying to survive even if there mutated and defrent.