Common Monsters
Some of the more common "monsters" players may find in a dungeon: (all information is from the AD&D Monster Manual 2e unless otherwise indicated)
Beholder
"A beholder, sometimes called a sphere of many eyes or an eye tyrant, was a large aberration normally found in the Underdark. These large, orb-shaped beings had ten eyestalks and one central eye, each containing powerful magic. Powerful and intelligent, beholders were among the greatest threats to the world." https://forgottenrealms.fandom.com/wiki/Beholder
Bugbears live in loose bands, and are typically found in the same areas as are goblins. Unlike their smaller cousins, however, these hairy giant goblins operate equally well in bright daylight or great darkness (as they have infravision to 607, so they are as likely to choose a habitation above ground as they are to select a subterranean abode.
Carrion crawlers strongly resemble a cross between a giant green cutworm and a huge cephalopod. They are usually found only in subterranean areas. The carrion crawler is, as its name implies, a scavenger, but this does not preclude aggressive attacks upon living creatures, for that insures a constant supply of corpses upon which to feed or for deposit of eggs. The head of the monster is well protected, but its body is only armor class 7. A carrion crawler moves quite rapidly on its multiple legs despite its bulk, and a wall or ceiling is as easily traveled as a floor, for each of the beast's feet are equipped with sharp claws which hold it fast. The head is equipped with 8 tentacles which flail at prey; each 2' long tentacle exudes a gummy secretion which when fresh, will paralyze opponents (save versus paralysis or it takes effect). As there are SO many tentacles with which to hit, and thus multiple chances of being paralyzed, these monsters are greatly feared.
Drider
"Driders were drow that had been transformed from the waist down so they had the body of a spider. The transformation was typically a punishment for failing a test of Lolth" https://forgottenrealms.fandom.com/wiki/Drider?so=search
Fomorians are the most hideous, deformed, and wicked of all giantkind. They tend to be solitary, dwelling in forlorn places such as mountain caves or abandoned mines. Their thick, hairy hides, combined with the pelts and odd metal bits they wear for protection, give an effective armor class of 3. Although these deformed monsters move but slowly for their size, they can do so with considerable stealth, and they have +1 to +3 on surprise depending upon the terrain.
A fomorian will have terrible deformities, such as one arm misplaced; huge feet on short legs; eyes on the side like a fish's or one in the back of the head; a hump on the back; a pointed, long head; flapping ears; a gaping or tiny mouth; a huge nose or snout; and so on. They have scattered patches of hair as tough as wire.
Shriekers are normally quiet, mindless fungus which are ambulatory. They live in dark places beneath the ground. Light within 30' or movement within 10' will cause them to emit a piercing shriek which lasts for 1-3 melee rounds. This noise has a 50% chance of attracting wandering monster each round thereafter. Purple warms and shambling mound; greatly prize shrieker as food.
Ghouls are ”undead,” once human creatures which feed on human and other corpses. Although their change from human to ghoul has deranged and destroyed their minds, ghouls have a terrible cunning which enables them to hunt their prey most effectively.
Ghouls attack by clawing with their filthy nails and with fangs. Their touch causes humans – including dwarves, gnomes, half-elves, and halflings, but excluding elves - to become rigid (paralysis) unless o saving throw versus paralyzation is successful. Any human killed by a ghoulish attack will become a ghoul unless blessed (or blessed and then resurrected). Ghoul packs always attack without fear. These creatures are subject to all attack forms except sleep and charm spells. They con be turned by clerics. The magic circle of protection from evil actually keeps these monsters completely at bay.
Lacedon: The lacedon is a marine form of the ghoul. It conforms in all other respects to ghouls.
Giant bats
have bodies up to three feet long, and their wings can spread between five and six feet wide. As you might guess, they find it hard to gather enough bugs to satisfy their larger appetites. They make meals out of small rodents and sometimes even attack larger animals like cattle, horses, and even people. They clamp onto their victim using tiny fingers on the elbows of their wings and tear out a few coin-sized bites.
All beetles are basically unintelligent and always hungry. They feed on virtually any form of organic material, including other sorts of beetles. They taste by means of their antennae or feelers; if the substance is organic, the beetle then proceeds to grasp it with its mandibles, crush it up, and eat it. Because of this thorough grinding, nothing actually eaten by giant beetles can be revived in any manner short of a wish. Beetles do not hear or see well, relying primarily on taste and feel.
Fire Beetle: The smallest of the giant beetles, fire beetles, nevertheless are capable of delivering serious damage with their powerful mandibles. They are found both above and below ground, being primarily nocturnal. Fire beetles have two glands above their eyes and one near the back of their abdomen which give off a red glow. Far this reason they are highly prized by miners and adventurers, as this luminosity will persist for from 1 - 6 days after the glands are removed from the beetle. The light shed illuminates a 10' radius.
These nasty creatures are found nearly everywhere. They are aggressive and rush forth to bite their prey, injecting poison into the wound, but in many cases this poison is weak and not fatal (add +4 to saving throw die roll). Also, as the centipede is small, it is less likely to resist attacks which allow it a saving throw (-1 on die). Centipedes come in many colors- pale gray to black, red to brown.
Found only in the fetid waters of swamps and marshes, giant leeches are a threat to any warm-blooded creature which passes within their movement range of 3". These horrors wait in the mud and slime far prey, and as it passes they strike. Giant leeches range from 1 to 4 hit dice in size; various sized creatures usually are found in the same group. The initial attack also attaches the sucker mouth of the giant leech, and on the next melee round, and on each round thereafter, it will drain blood the equivalent of 1 hit point damage per hit die it possesses. There i s only a 1% chance that the victim will be aware of the attack unless it comes out of the water, for the leech has anesthetizing saliva, and its bite and blood drain are not usually felt until weakness (loss of 50% of hit points) makes the victim aware that something is amiss. Giant leeches will come out of water at night to attack prey within 3". They can be killed by attack or with quantities of salt sprinkled on their bodies. There is a 50% chance that the bite of one of these creatures will cause disease which will be fatal in 2-5 weeks unless cured.