Sometimes in battles, I’ll see blue text saying “hidden skills unlocked” over the enemies’ heads.  For example, when I fight the dragon in the Dragon’s Lair, at the very end of the third round, the dragon will “inhale” (as it says in pink text) and “hidden skills unlocked” in blue text appears a split second later.  (Then the dragon gets killed.)  This happens every time I fight the dragon, so it’s not like once the skills are unlocked, they stay unlocked forever after.

What are “hidden skills” (in general), what unlocks them, and how can we see what specific hidden skills our specific enemies have just started using? They don’t seem to appear as buffs under their life meter or on the “info” screen.