class Animal < ActiveRecord::Base
end

class Bird < Animal
end

class Monkey < Animal
end

FactoryGirl.define do
  factory :animal do
    initialize_with do
      if [true, false].sample
        Monkey.new
      else
        Bird.new
      end
    end
  end
end

FactoryGirl.define do
  factory(:animal) do
    type { %w[Monkey Bird].sample }

    initialize_with do
      record = new(attributes)
      record.becomes(record.type.constantize)
    end
  end
end