Only three monsters have a Mega skill (Cryotan aoe mega freeze, Tankerion single target mega stun and Zimnyaya single target mega freeze) the other three have the mega effect in their special.. something to keep in mind if you are thinking of buying the chest.

Zimnaya - single target megafreeze.
Another note, there is a monster that have an aoe megapossession, the only one as far as I know, Opiuchus, which is an epic monster.

@phantaminum Edited !