When I was more active in MU* coding, how I'd handle mob behavior was one of this things I mulled on from time if I made my own game. As is, current MMOs tend to be either heavily scripted or little more than "spam this to keep a hidden number high so a mob focuses on you" sort of manipulation.
On the statistical end, I had affix behaviors like:
Confrontational = Focus on the character with the highest Attack/Strength.
Standoffish = Focus on the character with the highest HP/Vitality.
Deft = Focus on the character with the highest Dexterity.
Nimble = Focus on the character with the highest Agility.
Discerning = Focus on the character with the highest Magic Attack/Intelligence.
Zealous = Focus on the character with the highest Mind.
Flamboyant = Focus on the character with the highest Charisma.
These traits would be hidden to the player and would vary as things respawn, effectively making each fight potentially different. However, I upped the random factor further in that mobs could have further enhancements to the above mentioned categories. Depending on their move sets, you might not even notice, but a traditionally STR-heavy mob getting an even further STR bonus might facilitate a slightly different strategy. Especially since fights typically weren't going to be Many Allies vs. 1 Monster, but something more variable where I further distinguished mobs into size categories with party formations that followed.
Since this is an MMO and not text-based, however, we could technically go further in their (re)actions. Wear red when you fight a bull-type mob? You might be the target. Cast Ice magic against a mob that really hates the cold? It might wanna pick on you. Could even have ones that prefer hit and run tactics or might not even want to fight at all, instead turtling up and constantly curing themselves. Yoinking a page from FF4, anyone wearing metallic gear could receive slight debuffs simply being around certain mob types due to a magnetic aura. Some traits can be consistent, sure, as that's part of what makes a race/species/whatever, but I really do believe that if we want to make players distinguishable, our foes need to be, too. And not just because some want PvP that way.
Plus it kinda fits more snugly into my ideal crafting systems where items are more customized to our play style than every Iron Sword being exactly the same.