Here is my example...

gistfile1.txt

require 'rubygems'
require 'pry'

class Animal
  def speak
    puts "hello"
  end

  def self.create_animal(type)
    case type
    when "Dog"
      Dog.new
    when "Fish"
      Fish.new
    when "Owl"
      Owl.new
    else
      nil
    end
  end
end

class Dog < Animal
  attr_accessor :name
  attr_accessor :age

  # def initialize(name, age = 0)
  #   self.name = name
  #   self.age = age
  # end

  def speak
    puts "woof"
  end

  def dog_years
    self.age * 7
  end

  def self.family
    "Canidae"
  end

  def self.new_with_name(name)
    Dog.new(name)
  end

  def self.new_with_name_and_age(name)
    Dog.new(name, 10)
  end
end

# dog = Animal.create_animal("Dog")

# puts Dog.family

# dog = Dog.new_with_name("Buddy")
# dog = Dog.new_with_name_and_age("Buddy")

# bentley = Dog.new("Bentley", 5)
# puts "Hey #{bentley.name} #{bentley.dog_years}"

# izzy = Dog.new("Izzy", 9)
# puts "Hey #{izzy.name}"

class Fish < Animal
  attr_accessor :colour

  def speak
    puts "wibblewibblewibble"
  end
end

# bert = Fish.new
# bert.name = "Bert"
# bert.colour = "Orange"
# puts "Hey #{bert.name}, you're #{bert.colour}"

# ernie = Fish.new
# ernie.name = "Ernie"
# ernie.colour = "Orange"
# puts "Hey #{ernie.name}, you're #{ernie.colour}"

class Owl < Animal  
  attr_accessor :head_rotation

  def speak 
    puts "whoooo-ooo, whoooo-ooo"
  end
end

# harry = Owl.new
# harry.name = "Harry"
# harry.head_rotation = 180

# puts "Hey #{harry.name}, your head is #{harry.head_rotation} degrees"

print "What animal do you want to create? "
animal = Animal.create_animal(gets.strip.chomp)

unless animal.nil?
  puts animal.speak
end