Monsters do not need innovative mechanics to be interesting. The difference in the statistics between kobolds, goblins, orcs, hobgoblins, gnolls, bugbears and ogres in original D&D (and AD&D) is minimal. What makes them different? Culture, organisation, and story. This isn’t to say that monsters shouldn’t have interesting mechanics, but they’re not the only consideration.

With any monster, when I am designing a scenario, I would like to say “this is the only monster that fits that role.” That they’re identifiable and iconic enough in both your and your players’ minds that it makes sense that they are there. 

Dungeons & Dragons delights in having lots and lots of different monsters, but do you need them all in your campaign? You likely don’t. Especially when you are building up the cultures and settlements in your world. For the intelligent monsters that have their own cultures and homelands, pick a few that you want to tell stories about, and place their settlements on your map. Then ask yourself, what are they doing there? 

It is okay to give monsters alien desires. To give them beliefs and motives that do not fit with our own. They are not humans. The trick is that conveying that to the players in a consistent way. And yes, many of these groups are likely to be hostile to humanity. Some can never be brought around. Some might be able to be. But that sort of campaign is *usually* outside the regular D&D game. 

When you say “The gnolls of the Eastern Reaches are led by a savage priestess who encourages them to show their piety by collecting the ears of their victims, and only those with a thousand ears are guaranteed passage to the afterlife”, that gives you something to use in game. If you then have monster stats to back that up, you’ve got something that is reinforced and the players remember long after!

I once read in a history book about two European nations with similar cultures, the same religion, and beneficial trade alliances. They still went to war. That there is strife between all the creatures of a typical Dungeons & Dragons game? It isn’t that unbelievable once you know our history.

Consider Europe circa 300-500 AD. During that time, the Western Roman Empire collapsed, and the political map of Europe was rewritten.

One of the major pressures on Rome at that time was the migration of “barbarian” races into Europe. We tend to look at a D&D map and draw out the whole world with everything known. And, included in this, are the monster races. The DM and players know where they live. However, if you were to draw a map of 500 AD Europe, there would be question marks around the borders. 

Where did the Huns come from? We don’t know. They ravaged large parts of the Roman Empire, but they came from that big question mark to the east. And they drove the Gothic nations before them. 

Question marks on the Dungeons & Dragons maps are wonderful things. The borders of the unknown. Anything could live out there!