I'm 90% sure it's because you didn't close your table tags properly but without seeing your site, it's hard to tell. It looks like you're missing a </tr> tag, a </thead> tag, and a </table> tag. Also, you're not supposed to put <tr> tags inside <tr> tags.

<table class="list" role="grid">



   <thead>



       <table class ="animaltable">{foreach $[board]}



           {if !$[board.is_redirect]}



           <tr id="$[board.content_id]" class="$[board.content_class]"><td>

               <table>

                   <tr> <th class="animalcategory"></th> </tr>

                   <tr> <td class="animaltitleboard"><div class="animaltitle">$[board]</div></td><td class="animaldescriptionbox"><div class="animaldescription">$[board.description] </div></td> </tr>

                   <tr><td class="animalsubboardsbox"><div class="animalsubboards">{if $[board.sub_board]  > 0} {if $[board.sub_board] != 1}{/if}{foreach $[board.sub_board]}<a href="$[board.sub_board.href]">$[board.sub_board.name]</a>$[board.sub_board.comma] {/foreach} </div>{/if}</td></tr></table>

               </td>

               <td class="animalrecent">{if $[board.posts] > 0}



                   {if $[board.last_thread]}



                   $[board.last_thread.recent_link] by $[board.last_thread.last_post.created_by]



                   {/if}



                   {else}



                   {/if}</td> </tr> </table></thead>

       </table>

try using this and see if you have any better luck with it. it doesn't make my thread list weird, but it also doesn't style the subboards so this might need some further looking in to. edit: i'm dumb and didn't realize i was just looking at a thread list with no subboards. it works for me as far as i can tell o/