After 100/500 fights they wouldn't be in the same physical (or mental) condition anymore, even if they were identical before. They wouldn't look or behave the same either: one would have more muscle, or more battle scars, or might glance around nervously anticipating attack, or confidently seek out opponents, etc. Basically, an experienced/aggressive monster should be distinguishable from a less experienced/less aggressive one, not just in their description (which should be done better), but in behavior and appearance.
As far as the description goes, it would help to mention apparent age, physical condition, battle scars and behavior. Like "A young looking rat, shivering with fright and ready to bolt" vs. "A rat, seemingly in it's prime, it's battle scars a testament to many successful fights."
I doubt it could be easily implemented at this point, but personally, I think NPCs and player characters should be pretty much exactly the same to the server except that one is controlled by a script and the other is controlled by a client. For spawning (rats for example), have a rat make a nest, another rat finds it and wanders in, both adult rats stay in /near the nest, the nest produces baby rats, baby rats level up to adult rats, which then go make new nests. A player could destroy a nest, but it would be defended by two adults and lots of babies and might require special tools (like a shovel, axe, pickaxe, torch, whatever).