Perhaps, though, there can be a formula to determine spawn ratios of certain monsters based on the rapidity at which they have been killed recently, and the rapidity (theoretical) of which they reproduce. This would make it possible to overhunt, but not wipe out certain Mobs. It would also automatically make the mobs with useful or potentially expensive items rare to find. A useful side effect of this would be if species could be protected by a group of \'herdsmen\' (players) who would farm (herd them, or keep others away from them) the animals for their usefulness. Another cool part of this would be that large creatures such as dragons would come into knowledge (spawn) every once in a long while and require beating by a sufficiently brave group of adventurers.
Going a step further... a large, stretched step forward, the mobs could be \'born\' as young of their species and slowly mature until they become the stronger adults of their type. I see the biggest issue with this being that if a zone wasn\'t regularly hunted, or during a period of time where very little amount of people hunt them they would become a very strong group of monsters. A solution then would be a formula for the randomized death of the mob after a long period of time. It would be so spread out of a time that it wouldn\'t effect the normal hunting of the creatures, but it would occur every once and a while. As a safety measure for angry people, it would be impossible for it to die when engaged in battle or at other key times of its life.