The Tavern

Often, the enemies will behave stupidly. One example is this:
where #s are walls, Es are enemies, and @ is the player. The enemies will attack you one at a time from the top, instead of flanking around to the back so 2 enemies can attack you at the same time. Also, once I encountered cerebov in the following situation

For some reason, cerebov (&) kept hugging the lava and not moving instead of just going around it.

Smart AI leads to tedious behavior to outsmart them, meanwhile stupid AI leads to tedious behavior to exploit them, though the latter is usually less tedious.

Ranged AI and pack AI were both introduced with the idea of getting rid of behaviour a developer considered "degenerate" (using terrain and moving, respectively). Instead they had the effect of making monster AI more exploitable and less dangerous.
If you believe that you can design an AI smarter than your players in a game with anywhere near as much entropy as DCSS, then you're either incredibly delusional, or a developer for Zynga.

I cannot reproduce the Cerebov situation you describe. Are you sure he wasn't just casting spells instead of moving?

He cast a few spells, but refused to pursue when I left los, even after I waited a bunch of turns. I hugged the outside wall of his castle until I went around the lava, then I lured him into one of the loot rooms and killed him there. Also, what are pack and ranged ai?

I've had cerebov refuse to come out from behind his lava too, where I just pelted him with javelins. He occasionally cast spells, but seemed to spend most of his turns shuffling 1 tile to the left then back again. I've occasionally seen other monsters do the same thing, where they can't figure out how to get around some lava or water.

This is because he's patrolling and won't stray far from his original location after you're out of LOS (another AI "feature" that makes monsters less dangerous instead of more).
Pack AI is the way monsters try to stay near their pack leader, effectively slowing down pack monsters (yes I know swapping also slows them down, this is separate).
Ranged AI is how centaurs etc. try to stay a few squares away from the player when they come around corners or you use stairs, making them easier to escape and allowing some forms of dancing that wouldn't otherwise be possible.

Could you perchance explain how Cerebov not being easily led away from the vault containing his rune and other monsters makes him less dangerous?

Probably because you can more easily disengage to reset the fight

I guess it makes him less dangerous in the same way that speed 11 monsters are way less dangerous than speed 9 monsters. You only fight until you don't like how the fight is going, meaning you have to essentially decide to let the monster kill you to die to it. A monster that pursues the player isn't so easily disengaged.

With cerebov guarding a rune, there are two considerations. Difficulty of killing cerebov (thus making the rune not guarded), and difficulty of getting the run without killing cerebov. The patrol behavior makes number 2 harder, but seems to make number 1 easier under some circumstances.

Unique Pan/Hell lords are also not the only patrolling monsters in the game.