A fantasy campaign traditionally (though not necessarily) includes monsters in its population.  These monsters are of three basic types:  humanoids, nonhumanoids, and fantastic beings.

Humanoids are the fodder of most worlds.  Orcs, goblins, trolls, ogres, fuzzy cat people, whatever… these are the bread and butter of the adventurer on the line, and they come in sufficiently organized numbers so as to pose a threat of invasion, giving the humans an excuse to hate them.  If you are concerned with realism in your game design, humanoid tribes will have much the same concerns as human civilizations, primarily food, terrain control, etc.  Relations with the humans will typically be exasperated at the start of the campaign, explained by factors like an inability to communicate well, vastly different ethics, and plain old competition for resources.  However, the humanoid monsters generally do not possess the potential for personal prowess that humans have, as evidenced by the fact that human adventurers tend to kill them in great numbers.  We can thus infer that the typical humanoid lacks sufficient technology to allow him to progress, and is locked into a more or less nomadic hunter-gatherer tribal existence.  They probably do not have a high degree of metallurgy and use bronze and stone for weapons, and hides, wood and bronze for armor.  Their lack of agriculture makes them wander around in search of food, bringing them into conflict with territorial humans.  Lack of agriculture also makes them less likely to have the leisure time humans have, and so they are slow to develop new technologies, including magic.  Given all of these disadvantages, they have to have some sort of compensation to avoid being wiped out offhand by the humans.  The simplest advantage is faster breeding, the single most important evolutionary trait there is.  This also increases their need to take more land for foraging, putting them into increasing conflict with humans.  Other advantages may take the form of military readiness:  with no farmers, every humanoid is a hunter, and capable of fighting.  Hunting societies tend to breed for warrior traits and establish warrior ethics as well.  However, it still all comes down to food, and food is what will cause conflict between the player society and the roving packs of brutal humanoids.

Something to note about an organized group of hunter-gatherer humanoids is that they need really large amounts of land to support themselves.  A realistic figure for human hunter-gatherers is no more than 2 per square mile, or else you run into environmental degradation problems.  Obviously, having to run a mile to encounter 2 humanoids makes for a very dull game, and requires a gigantic landmass if you have 1000 player characters all running after them simultaneously.  To some extent, you can make up for the implications of denser populations in the wilderness through explanations like raiding the fertile human territories for food, fishing, and the like, but ultimately you should have an accelerated food chain.  Superwheat may not be the only hyperefficient crop available, and you could say that the humanoids are omnivorous, taking a good deal of sustenance from high-yield vegetation which is unpalatable to humans like swamp weeds, mosses, lichens, insects, grubs, etc.  In turn, these nonhuman food sources can also support a larger number of edible herbivores, who can in turn support more carnivores and omnivores per square mile.  It may require a bit of black boxing to bring a reasonable ecology up to the point where it becomes fun for players.

Nonhumanoids comprises monsters that are more akin to animals, like giant spiders, serpents, stirges, etc.  In order to keep these believable, you obviously don’t need a high degree of societal understanding, but the ecology does have to make sense.  One forest with nothing but carnivorous wolf-things is not a viable situation.  Dangerous carnivorous creatures cannot congregate successfully in small areas (unless the carnivores themselves are very small compared to their prey).  They require a certain number of herbivores to harvest, while leaving enough herbivores to replenish the stock.  The herbivores in turn feed on plants, including plants that are not normally edible by humans.  Omnivores can exist in slightly greater numbers with the same supporting herbivore population, but their numbers will still be a tiny fraction of all the animal life in the area.  Once again, you can accelerate the food chain to allow for more food sources for nonhuman monsters, just as you do for humanoids.

Fantastic creatures do not have to follow the rules of reality.  These are the undead, dragons, daemons, miscellaneous energy beings, and what have you.  It still behooves the world builder to figure out why these creatures are where they are, and what resources they might require.  A Tolkienesque dragon is a gigantic carnivore, and a flying one to boot, increasing its calrie requirements.  Logically, one decent-sized dragon would strip an area of all animals and people very quickly, then move on to the next area, which is a good reason to try to kill them, but it disallows the concept of the dragon hoard (which is fine if you want to go that route).  All fantastic creatures are black boxes by default, and as such the designer can apply black box logic to them.  A dragon is indeed huge and hungry, but maybe it has a 20-year sleep cycle unless prematurely awoken, and when it is awake it does eat everything it can before going back to sleep.  Vampiric undead may also have long periods of torpor, reducing energy requirements, and have "extra-dimensional" sustenance as well, but they need to hang on the fringes of civilization so they can occasionally drink blood, which contains some element they have a deficiency in.  A daemon may not be a native inhabitant of the world, appearing only when summoned or due to "dimensional rift/convergence," and they are very unhappy whenever this happens.  As long as you even make an attempt to apply logic to these patently illogical creatures, it will be appreciated by the discerning player.