I'm not sure I remember correctly, but I think a certain, early Ravenloft Monster Manual featured a creature (some kind of a goblin?) that had negative HP, as in it operated (lived) with negative HP, and actions that would have hurt other monsters and enemies actually healed it, and, in turn, it was hurt by healing.
Has this concept been adopted to later editions of D&D? If so, what monsters / creatures have negative HP, and where can I find rules for such a twist?
What you describe sounds like the Nilbog, which is the oh-so-clever "Goblin" spelled backwards. Wikipedia and DnDWiki say it originates from White Dwarf, which seems about as crazy as the creature itself. As far as I'm aware, it first appeared in the original AD&D Fiend Folio (p. 67 of my copy). The two above entries both reference the Llort, which is, of course, "Troll" spelled backwards. * groan * I haven't been able to find any monster entry for that one, though.
Necromancer Games made an update for D&D 3.x and Pathfinder. You can find the Pathfinder version online here or pick up the Tome of Horrors Complete.
As far as I am aware, no creature in 3.5 has this sort of feature; Wizards of the Coast went out of its way to make sure that numbers always went in the same direction (namely, higher = better).
However, Undead have the trait of being healed by certain forms of negative energy damage, and harmed by certain forms of positive energy healing. That certainly does not encompass all forms of damage (or even all forms of negative energy damage), though.
I can’t speak for other editions, though, so this is an incomplete answer.
