I use this tag in my game to indicate monsters which are meant to be encountered solo but are more impressive or capable than a legendary version. At a minimum, a monster with the mythic tag has maximum hit points and legendary actions. It can be applied to any creature of any CR.
Just as mythic heroes can call upon power beyond reckoning, mythic monsters are greater than others of their kind. Some are empowered by deities or great magic and sent into the world to sow ruin and reap destruction. Others are instead relics of a bygone age when the power of creation itself flowed through the veins of every living being. Though not necessarily malevolent, these ancient creatures are a force to be respected and feared.