dynamically creating a subclass

[Gribouillis]

If you really need to convert strings, you could use a metaclass to collect the subclasses

class PetMeta(type):
    pet_classes = {}

    def __new__(cls, clsname, superclasses, attributedict):
        if clsname in cls.pet_classes:
            raise ValueError(
                "Redefinition of Pet subclass '{}'".format(clsname))
        tp = type.__new__(cls, clsname, superclasses, attributedict)
        cls.pet_classes[clsname] = tp
        return tp

def pet_class(name):
    return PetMeta.pet_classes[name]

class Person:
    def __init__(self, pet_types):
        self.pets = [pet_class(t)() for t in pet_types]

class Pet(metaclass=PetMeta): pass
class Dog(Pet): pass
class Cat(Pet): pass
class Bird(Pet): pass

john = Person(['Dog', 'Cat'])
print(john.pets)

The drawback of __subclasses__() is that it only knows direct subclasses.