<?php
    $category = get_terms( 'series' );
    $catID = $category[0]->term_id;
    $args = array(
        'numberposts' => 5,
        'category' => $catID,
        'post_type' => 'anime'
    );
    $catPosts = get_posts( $args );

    echo '<form method="POST">';
    echo '<select name="goToPost" onchange="document.location=this.value">';
    echo '<option value="">'.__('Relevent Posts', 'your_text_domain').'</option>';
    foreach( $catPosts as $singlePost ) {
    echo '<option value="'.get_bloginfo('url').'/index.php?p='.$singlePost->ID.'">'.$singlePost->post_title.'</option>';
    };
    echo '</select>';
    echo '</form>';
?>