Dragon is not defined

import random

COLORS = ['yellow', 'red', 'blue', 'green']


class Monster:
  minHitpoints = 1
  maxHitpoints = 1
  minExperience = 1
  maxExperience = 1
  weapon = 'sword'
  sound = 'roar'

  def __init__(self, **kwargs):
    self.hitpoints = random.randint(self.minHitpoints, self.maxHitpoints)
    self.experience = random.randint(self.minExperience, self.maxExperience)
    self.color = random.choice(COLORS)

    for key, value in kwargs.items():
      setattr(self, key, value)

  def __str__(self):
    return '{} {}, HP: {}, XP: {}'.format(self.color.title(), 
                                          self.__class__.__name__, 
                                          self.hitpoints, 
                                          self.experience)

  def battlecry(self):
      return self.sound.upper()




class Goblin(Monster):
  maxHitpoints = 3
  maxExperience = 2
  sound = 'squel'

class Troll(Monster):
  minHitpoints = 3
  maxHitpoints = 5
  minExperience = 2
  maxExperience = 6
  sound = 'growl'

class Dragon(Monster):
  minHitpoints = 5
  maxHitpoints = 10
  minExperience = 6
  maxExperience = 10
  sound = 'raaaaaaaaaar'