Tsathoggua is a big frog from outer space.  I think I can wrap my mind around that.

I just had an idea, playing God of War:

Usually, only PCs can get negative hit points and everything else is automatically dead, when it has no more hit points. However, sometimes it will happen that an enemy is at exactly 0 hit points. At that point, I would treat them as unable to attack or stand, or really do anything than speaking with difficulty. It's then up to the players what to do with the dying that are still around.
They can finish them off after the fight is over, patch them up so they will likely live, or just leave them where they fell, almost certainly to die.

I've never seen it in an RPG, probably because it's widely considered too gory. But sword & sorcery is either cartoonish (like God of War), or battle and death is supposed to be highly unpleasant and nasty. I would personally not get too much into detail with the bleeding and mutilated corpses on the ground, but I like the idea of occasionally reminding players that hacking at people with swords and shoting them with arrows isn't nice clean instant death.

Blackadder23 wrote:

In tribute to Mr. Robert E. Howard, I'm always extremely graphic about describing the mayhem during combat: "Your sword blow splits the brigand's skull to his teeth, splashing blood and brains like water" or "Your axe severs the ape-man's arm with an enormous jet of blood.  The creature staggers around for a few moments, clutching the wound, and then stumbles to the ground to lie moaning before finally expiring" or "You plunge your spear into the mountain ape's abdomen.  As you wrench your weapon loose, the monster bellows and bloody entrails gush from the gaping wound".  Also, nothing just dies if it can dissolve horribly (shrieking all the while), and nothing dissolves horribly if it can explode instead.

Its probably hard to describe the mayhem created by medieval weapons as graphically as it deserves.  I usually reserve descriptions for crits / massive damage or for opponants teatering on the edge of defeat.  Great flavor text above however.
 

That creatures other than nonsentient undead fight until the last of them is a kind of nonsense in RPGs that is not only annoying in respect to sword & sorcery.
Animals fight to live, not to kill. Survival always is a priority over killing your opponent, except for situations where it "benefits the population" to sacrifice yourself. And that situation really just applies to insects, fish, some reptiles, and other very simple life forms, in which the adults take no active role in increasing the survival chance of the young. For most large animals, instinct dictates to abandon the young and try again next year, rather than getting killed to maybe stop a predator.
Situations where sacrifice is absolutely necessary are extremely rare, and the rest of the time creatures want to survive. When it becomes clear that winning is impossible and you won't be able to drag out the fight long enough to benefit your allies in some way, enemies will try to get out alive, whatever it takes.
Even morale checks are for situations when the fight has not yet become hopeless, but the enemy might get spooked anyway. Even if they make all their morale checks, there comes a point where a tactical retreat or offer of surrender becomes necessary.

However, there are many good reasons why you wouldn't want defeated enemies to escape. They could regroup or heal and become a problem later, which can now be quite easily avoided. And then there is also always the chance that they raise alarm or get reinforcements, which in a dungeon is almost sure to happen.
My next game will be an assault on a cult hideout. Now I really want to see what the players will do when in the middle of a fight one enemy turns around and runs deeper into the caves.