Supplement the abstract class so that classes inheriting from it don't have to override the method.
The following code must work:
class Cat(name: String, breed: String) extends Pet(name, breed):
def getKittysName: String = "Cat's name is " + super.getFullName
val cat = Cat("Tom", "British Shorthair")
cat.getKittysName // Cat's name is Tom it's breed is British Shorthair