Creature

A creature, in terms of game mechanisms, could be any object with a non-depleted Health bar, otherwise it would have been called a corpse.

Upon death
Quite naturally, only corporeal creatures leave a corpse behind them (not all of them are exploitable though). While spirits only grants half of the normal benefit for Soul Reaping upon their destruction (among other numerous differences)... leaving a doubt about their real essence (as full-fledged creatures).