Understanding Ruby Modularization

$rubymain.rb
Traceback (most recent call last):
        3:from main.rb:1:in`<main>'
        2:from/home/ec2-user/.rvm/rubies/ruby-2.5.3/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in`require'
        1:from/home/ec2-user/.rvm/rubies/ruby-2.5.3/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in`require'
/home/ec2-user/environment/human.rb:3:in`<top(required)>':uninitialized constant Animal(NameError)

require"./human"

# Creating an Instance
tanaka=Human.new ("Taro Tanaka", 25, "Train")
suzuki=Human.new ("Jiro Suzuki", 30, "baseball")
sato=Human.new ("Sato Hanako", 20, "Movie")

# Run Method
tanaka.say
tanaka.think
suzuki.say
suzuki.think
sato.say
sato.think

require"./thinkable"

class Human<Animal
    include Thinkable

    # set what variables to use in an object
    attr_accessor:hobby

    # Special Method for Initializing Instance Values
    default initialize (name, age, hobby)
        self.name = name
        self.age=age
        self.hobby=hobby
    end

end

#Defining a Class
class Animal
# Object Variables
    attr_accessor:name, :age

# method
def say
    puts"#{self.name}.# {self.age} years old."
end 

end
= begin
   default initialize(name,age)
    self.name = name
    self.age=age
  end
=end

= begin
animal=Animal.new ('Taro Tanaka', 25)  
animal.say
=end

#Defining a Class
class Animal
# Object Variables
    attr_accessor:name, :age

# method
def say
    puts"#{self.name}.# {self.age} years old."
end 

end