creating multiple objects of the same model in a single form

I want to create multiple instances of the same object in one form. I have tried

<%= form_tag(animals_path) do %>
 <% @animals.each do |animal| %>
  <%= fields_for "animals[]",animal do |f| %>
   `//animal fields
 <% end %>
<%= submit_tag 'save' %>`

but got error: "undefined method `merge!' for nil:NilClass"

I have built @animals at my controller.

using rails 3.0.7

any help is appreciated.