huge.rb

class Foo
  def huge
    puts "huge"
  end
end

module Bar
  def new_huge
    puts "small"
  end

  def self.included(base)
    base.send(:alias_method, :original_huge, :huge)
    base.send(:alias_method, :huge, :new_huge)
  end
end

Foo.send(:include, Bar)

Foo.new.huge