Monstrous Flaws

Give each critter a monstrous flaw, the thing that defines them as an enemy or monstrous creature. Have mechanical effects tied to it, and provide reasoning for why that flaw makes the creature monstrous.

Goblins: Suspicion. Goblin societies can't get anywhere because no goblin can trust another. They backstab and bicker, coming together in warbands and clans only by dint of violence or greed, then perhaps accomplish one goal and immediately devolve into infighting again. The only thing that keeps them together is that goblins mistrust the outside world even more than their kin. Their Suspicion trait makes them resistant to charm effects & grants a bonus on Initiative.

Ogres: Hunger.

Giants: Arrogance.

Dragons: Greed.

Orcs: Anger.

Beholders: Paranoia.

Fiends: A wide variety: Wrath for demons, Pride for devils. Some lust for succubi, etc.

Undead: Bitterness. They resent the living for their warmth.

Kobolds: Cowardice.