Magento released a patch for magento 1.9.3.x to be able to support php7.2. The patch includes /lib/mcrypt_compat/mcrypt.php so that php7.2 which does not have the mcrypt module (at least officially), can run without problems.
However i just notice that Magento 1.9.3.10 actually does not include this mcrypt lib, which means you either have to install mcrypt for php7.2 with pear, extract the file from a Magento 1.9.3.9 installation or use php7.1 which has mcrypt.
Does anyone know why this is the case?