Change the animal model to be instantiate with an empty array of toys, you can then add toys to the array: 
class Animal
    attr_accessor :name, :age, :gender, :species, :toys
    def initialize(name, age, gender, species)
        @name = name
        @age = age.to_i
        @gender = gender
        @species = species
        @toys = []
    end
end