It's fun to put NPC wealth in very difficult to carry around things that they would actually have. Doesn't make much sense for everyone to carry the majority of their wealth in nice large sum packets.
Libraries for wizards, for example. In a recent game, the PCs killed a pesky wizard, and I put half of his wealth into very large cumbersome books. The PCs picked out a few and then left the rest.
Statues are a good one for egocentric enemies. Up-play how well carved and what nice marble (or whatever) it is. Then watch the PCs try to drag the 2000 pound statue back to town! And then try to actually sell it...
Mostly, I would think an NPC can equip himself as well as PCs can, barring unusual circumstanaces. If PCs can go the ye olde magic shoppe and trade in their +4 tower shield of fire protection for a +3 keen scimitar, then the NPCs can do the same thing. If the PCs have to scrape by, creating magic items themselves or traveling to find someone to make them for them, then the NPCs should be bound by this as well.
Certain enemies I would say always have a ready supply of what they want. Nobles would have an easier time, as would the pit fiend in the OP. Those of little means would, also, have less ability to trade than the PCs, for example a tribe of orcs who can only trade through certain venues.
I havn't used random treasure generators in years. I pick things that make sense based on what I know about the area, enemy, world, etc.