<?php
// Insert into your functions.php and have fun by creating login error msgs
function guwp_error_msgs() { 
  // insert how many msgs you want as array items. it will be shown randomly (html is allowed)
  $custom_error_msgs = [
    '<strong>YOU</strong> SHALL NOT PASS!',
    '<strong>HEY!</strong> GET OUT OF HERE!',
  ];
  // get and returns a random array item to show as the error msg
  return $custom_error_msgs[array_rand($custom_error_msgs)];
}
add_filter( 'login_errors', 'guwp_error_msgs' );