fetch_feed: retrieve entries in the appearing order, not chronologically

Ok, found. I spent hours on this but I managed to find the solution.
The command I was looking for was $rss->enable_order_by_date(false);.

So you should set (for benefit of the community):

    <?php
    /* include the required file */     include_once(ABSPATH . WPINC . '/feed.php');
    /* specify the source feed   */     $rss = fetch_feed('FEED_URL');
    /* disable order by date     */     $rss->enable_order_by_date(false);
    /* checks obj. is created    */     if (!is_wp_error( $rss ) ) :
    /* specify number of items   */     $maxitems = $rss->get_item_quantity(X);
    /* create an array of items  */     $rss_items = $rss->get_items(0, $maxitems);
endif;
    ?>
    <ul>
        <?php if ($maxitems == 0) echo '<li>Content not available.</li>';
    else
        // Loop through each feed item and display each item as a hyperlink.
        foreach ( $rss_items as $item ) : ?>
            <li>
                <a href="https://wordpress.stackexchange.com/questions/84325/<?php echo $item->get_permalink(); ?>" title="<?php echo esc_html( $item->get_title() ); ?>" rel="external"><?php echo esc_html( $item->get_title() ); ?></a>
            </li>
            <?php endforeach; ?>
    </ul>

Hope this helps! 🙂