I like to give monsters potions, then have them drink the potions at the start of the encounter. The potions count as the encounter’s treasure.
Once, in a 3e combat encounter, I gave a ghost a special ring of regeneration that deals negative energy that heals undead. Essentially, it’s a ghost touch ring that hits the wearer with an inflict every round.
The encounter was very long and boring. The players didn’t know where it was getting so many hit points. They calculated that it had taken more damage than a CR-appropriate ghost could possibly have if its hit dice were maximized.
The ring was very expensive.
It counted as the encounter’s treasure.