One of the most fun things about D&D and other roleplaying games is and has always been facing a wide variety of horrible and imaginative creatures. Monsters have been inspired by folklore and legends (such as vampires, dragons or minotaurs,) by works of genre fiction (such as orcs, or all manner of Lovecraftian horrors,) or from the twisted minds of game designers or DMs (Beholders, mind-flayers, and rust-monsters). Regardless of their origins, players love to encounter new and exciting creatures to outwit, outfight, or outmaneuver, and DMs love to pit their friends up against strange and imaginative creatures.
There is no shortage of choice for the DM looking for creatures to use in their latest adventure. With three Monster Manuals currently available, a Dark Sun-specific monster supplement, and numerous creatures presented in adventures or sourcebooks like Draconomicon, Open Grave, or Demonomicon, there are literally thousands of creatures ready-made to drop into your dungeon. The Monster Builder makes it even easier to make your own creatures and modify existing critters to suit your every imaginable need. Additional monsters are almost never unwelcome additions to the game. Like Jello, there’s always room for more.
On the other hand, while there may always be room in D&D as a whole for monsters, that doesn’t mean that you as a DM are obligated to include every creature in every source as part of your campaign world. Some creatures, like those in the Dark Sun Creature Catalog, are explicitly designed for a specific setting—while they can certainly be used in others without too much disruption, there’s no reason to assume that they exist by default. In fact, the choice of what types of monsters you include in your campaign can be a defining factor for your game. You may choose creatures to support a particular theme of your game, for example, or to suit the particular environments in which your campaign takes place. You may also want to select monsters to give your campaign world a specific feel, be it by omitting creatures common to other settings or including new monsters unique to your setting.
Choosing based on a campaign theme is fairly easy. If your campaign has a strong theme of fighting the undead, for example, you’re going to want lots of skeletons, zombies, and vampires. If, on the other hand, you choose to focus on the fey, you might lean towards hags, eladrin, gnomes, and fomorians.
Similarly, choosing environmentally appropriate creatures is relatively simple. A campaign set underwater for large portions probably isn’t going to feature a large number of fire-based creatures, for example. Don’t be afraid to play against type, though—while you might expect a frozen wasteland to be populated largely by ice or cold-based creatures, a tribe of goblins that have enslaved a number of minor fire elementals may be a very interesting threat, and can lead to interesting encounters.
Choosing based on what fits your vision of the campaign world is easy enough as well, since it is entirely in your hands. My current campaign world has no orcs or gnolls, for example, and dragons are so rare that most people believe they are myths. Goblins and drakes are relatively common, however, and my players have had repeated run-ins with a race of small, pale darkness-dwelling creatures with power over shadows. Creatures unique to the setting can be very useful in defining it. Draconians and kender would feel out of place in Dark Sun, but it’s hard to imagine Dragonlance without them. Another element of this type of creature selection is that you’re free to declare that a certain monster family simply doesn’t exist if you feel it’s too dumb—and D&D has definitely had its share of lame creatures throughout its history. If you don’t like owlbears, or think the tarrasque doesn’t make sense, then you can certainly create a campaign setting where such things never existed.
Be careful not to exclusively rely on one type of monster exclusively—it can be dull to fight the same thing time after time, and many monsters of given “types” share resistances and vulnerabilities that can make combat much easier or more difficult depending on how your players have built their characters. A party with no reliable source of radiant damage, or who rely heavily on poison or necrotic damage, for example, will find facing hordes of undead to be a much less pleasant prospect than other groups might. You may want to warn your players not to pick too many powers of a given damage type if you’re planning on relying heavily on a type of creature that resists it, and you may need to fill out encounters with creatures of different types if your party has a high proportion of abilities that exploit those vulnerabilities.
Another key thing to remember is that you can always “reskin” creatures to serve your purposes. I mentioned above that my campaign world has no gnolls, but that doesn’t mean I haven’t used them. I ran a few encounters recently where my party battled a group of cultists who had been magically transformed into “beast-men”—all of whom used gnoll statblocks. I’ve also run encounters where I have reflavored a variety of disparate creatures into a unified theme—leveled up versions of spiretop drakes, fire beetles, and a very heavily re-imagined dragonborn became a group of lightning elemental creatures guarding the power core of an ancient lost civilization, with the drakes acting as lightning motes, the beetles as medium-sized lightning creatures, and the dragonborn becoming a large lightning-breathing drake.
However you decide to select the monsters you use in your game, remember that that choice can communicate a lot about the game world or the themes of your campaign. Don’t be afraid to reflavor as needed, but never forget that variety is the spice of life; even if you’ve been reflavoring wildly, your players see what you tell them they’re fighting—and if it’s demons or zombies every time, no matter what’s going on under the hood they may become bored. Even with a strong theme to your game, never be afraid to throw in something different just for fun from time to time. That is what the game is about, after all.