Lingering may be appropriate for ghosts, swarms, slimes, etc. Regeneration is for creatures with heavy duty supernatural healing. Use both for a nightmare creature that will make your players throttle you.
Lingering: When this creature would lose a hit die in combat, instead reroll it. If the result is a one, it loses it as usual.--
Lingering Hardcore: And for a small, hydra-esque twist, if the rerolled die comes up as a certain number, like say a six, the creature also gets to roll a bonus hit die too!
Regenerating: On each of its turns, this creature rerolls a previously lost hit die unless it took damage from its vunerablities list within the last round.
Lastly, I know my art up there kinda sucks (those are supposed to be ghostly carp up there), but in my defense I am out of practice, photographed it straight out of a moleskin, and made it without any penciling first.