A creature, in terms of game mechanisms, could be any object with a non-depleted Health bar.
Creatures are typically classified as either an ally or a foe. However, some special NPCs (usually service-providing NPCs) are considered neither -- they are not valid targets for skills that target either ally or foe. Some of those "neutral" NPCs can still be damaged by AOE skills not targetted at them though.
Health depletion
Quite naturally, only corporeal creatures leave a corpse behind them (not all of them are exploitable though). The death of any non-Spirit creature will trigger the effects of Soul Reaping, provided that conditions are met for the energy gain.
Notes
- Spirits issued from Nature Rituals generally affect all non-spirit creatures within their range.
- It's common that beneficial and non-damaging skills do not affect spirits at all, even if stated creatures, foes or allies in the skill description.
- Neutral wildlife such as the bull-like beasts in Sunqua Vale are non-targetable (and don't appear as minimap dots) until angered but are still part of the big family.
See also
Nearly every creature encountered in the game can be found in the Bestiary.