Add Custom Class To Last Post In A Loop WordPress:

Put this below code in Function.php File and save.
functionlast_post_class($classes){ global $wp_query; if(($wp_query->current_post+1) == $wp_query->post_count) $classes[] = 'Your Class Name'; return $classes; } add_filter('post_class', 'last_post_class');
After this you display your class name in your last Post.