There are many a tool that a successful dungeon explorer may make use of and one that is not ignored by the ruthless and uncaring are animals.
The original AD&D PHB has a list of common animals adventurers could buy and was certainly set up for homesteading but they also offered some use in dungeon exploring.
Here are some recommended uses for common livestock for the heartless dungeoneer.
Chickens: chickens are good eating, don't forget that a live chicken stays fresh longer a good bit longer than a dead one.
Chickens may seem to be noisy and a detriment to dungeon exploring but once removed from the light they start to become quiet and calm down. A chicken can be carried lightly bundled with it's head covered and it will not make much noise.
A chicken tossed into a doorway or around a corner will certainly serve to surprise what ever lurks beyond the threshold and may get all the attention of the beasts beyond thus giving the adventurers more time to launch an attack or observe the presence of monsters without immediate threat of attack, the chicken probably isn't long for this world when used in such a manner.
Cows and Oxen: Did you know you can ride a bovine? They are a slow but steady steed that isn't ill served for wilderness travel as long as one isn't in a hurry and doesn't mind being ridiculed by virtually everyone they meet. That said they are of limited use in dungeon settings will they will react poorly to the darkness, strange smells and simply refuse to go down stairs as cows will generally go up stairs without much struggle but are darned resistant to going down stairs.
Dogs: Guard dogs, hunting dogs and guard dogs seem a great companion animal in dungeons. Dogs can be fearsome combatants, have an excellent sense of smell and are loyal. They however are noisy, loud and react to things in a different manner from men and demihumans; the PCs may want to parlay with the molemen but it's really tricky to do that when one is hanging from your guard dog's maw.
Dogs require constant supervision in a dungeon environment and can't climb anywhere near as well as a man can so their mobility will limit a party that finds itself climbing ropes and ladders.
A reaction roll should be made for the parties dogs whenever a new group of men, humanoids or monsters is encountered. Dogs may note the presence of other beings long before PCs do but they may react by cowering, barking or even running away. Each round a dog barks may result in increased rolls for wandering monsters.
Donkey: capable pack animals men may be able to con into going into a dungeon. They will be more difficult than normal in a dungeon and prone to bolting with supplies and treasure on their backs.
Goat: they can be used like chickens but are heavier and not as easilt kept quiet. A bleating goat is likely to attract monsters in the dungeon so it may be useful bait but otherwise is a likely detriment.
Hawks: keep them out of dungeons they will be prone to react poorly and slowly in dungeon depths.
Horses: a horse may be dumb but they aren't dumb enough to go in a dungeon. the amount of things a horse fears are high and the dungeon is just full of them.
Mule: mules are even more caapble pack animals than are donkeys but they are also larger and harder to get to cooperate when they think they are in risk. Not recommended for use underground.
Pigeon: a pigeon can be used to send a message from a short distance inside a dungeon or readily from inside a castle if they are of the homing variety (and much higher expense), otherwise they are a quieter smaller chicken for all intents and purpose.
Pigs and Piglets: These animals are smart, horrible pack animals and tasty. Leave them on the farm or bring them along to use as food in the wilderness if your party is large enough but leave them out of the dungeon.
Sheep: If you see a sheep in the dungeon some kobolds just stole it or it's an illusion, be careful.
Songbirds: Tiny delicate birds have been used for centuries by miners as an alarm against noxious gases. A songbird will react to a smaller portion of gas than is required to harm a man or demi-human and will react to gases, mists and other air born threats earlier. A songbird must save vs a noxious airborne threat 20-50' further away than men and less sensitive beasts. If folks are not actively monitoring the birds they will fail to notice them swoon or die on a 5 or 6 in d6 roll. Some birds will swoon if gases are harmful to them but not all that harmful to men so sometimes this tactic is of limited use.
If one doesn't charm a songbird with magic a cage is needed.
Suggested cost for cages:
small wood cage 8 c.p. 1/4 lb
small brass cage 3 g.p. 1/2 lb
large wood cage 2 s.p. 1 lb
large brass cage 8 g.p. 3 lb