each_with_index

.row
- @pets.each_with_index do |pet, index|
  - index = index += 1
  - if index % 7 == 0
    .small-12.medium-4.large-3.columns
      .panel.card-style.centered
        <script async src="//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
        <!-- Pet Page -->
        <ins class="adsbygoogle"
          style="display:inline-block;width:250px;height:250px"
          data-ad-client="ca-pub-55555555"
          data-ad-slot="55555"></ins>
        <script>
        (adsbygoogle = window.adsbygoogle || []).push({});
        </script>