Hey.
Afaik a few elite unique items, like Griffon, DF, Death web, etc can only dropped by Baal, Diablo, and Uniques and Champions in LvL85 areas.
Is it about Item level, so an elite monster at lvl85 area is lvl86-87 and then he can drop?
If yes, does it mean in a terrorized area to lvl87 any random monster can drop these?
Or these drops refer more to other hidden stats, something about treasure value or w/e ?

yes, its about ilvl. treasure class determines what item type can be dropped as far as i remember. but the rule of thumb is: if you wanna find high class unique items like griffon, df etc. go farm high class terror zones. even normal monsters can drop such uniques then if its high enough.

cheers - enjoy the game

its a lot more complex than just ilvl or tc
look here if you really want do dive into it:
https://diablo2.diablowiki.net/Item_Generation_Tutorial