Do the monsters in your game "level up"?
I don't mean, do they get harder as you go deeper.
I don't mean, do they necessarily have a formal system for improving.
I'm wondering if they get better if they survive contact with the PCs.
After the PCs roll through an area, of course, some encounters change.
Some battle-scarred survivors will change their tactics. They'll build barricades to stop those shield rushes and put out water buckets to deal with fire apells. They'll spread out to stop area attacks. They'll get reinforcements if they can, patch up the wounded, and otherwise get set. They might team up with other monsters who also had problems going it alone against the invaders.
Other monsters will move on, going to a new area. Or they might be gone in the sense that something comes and eats them.
But I like the idea that some monsters learn not only in the "better tactics" or "get the hell out of Dodge" sense but in the "my sword skill went up" sense.
I have a few wiley monsters that have actually survived multiple encounters with the PCs. Some at range, some close in (there isn't always time to finish the wounded).
So for them, if they survived in some impressive or memorable fashion, I'll just promote them. A Hobgoblin Warrior might become a Tough Hobgoblin Warrior. A normal gargoyle might become Determined (yes, I love prefixes). On a smaller scale, I might add a perk to a given group (they all had Shield Wall Training, now the survivors learned Teamwork), or a skill (they take up Crossbow since you were last there).
If it was more happenstance, I'll be a little less generous. Like in the case of a distant skirmish, say, or a brief encounter that ended with both sides moving on, or "knocked out and left for dead." These guys might learn something, but not "promote" or level up.
A monster that flees a lot (and makes it) might get better at fleeing, via a point of Running or Aerobatics.
A spellcaster might learn a new spell.
A warrior might turn into a berserker after that time the PCs crippled his limbs and left him for dead. Technically, this reduces his value as a character but it's a positive change in his threat level.
So yes, in my games, monsters can and sometimes do "level up." They can promote up the scale, get better, and pick up abilities. It's not formalized (yet), but it's there. How about in your games?