Tower of the Archmage is hosting a posting about critters. Strange Animals to be specific and it is asking for a link to posts left in comments.
Here's my entry, the Monkey Spider.
Monkey spiders are small pestiferous creatures found in areas frequented by those who work with or store magical materials especially scrolls. Monkey spiders also known as shrew spiders, skitterpaws and scroll shredders, are neither monkeys nor spiders. Monkey spiders are tiny monotremes an inch to two inches long. The six-legged Monkey spiders also have a tiny prehensile tail they use as a seventh grasping limb. Monkey Spiders have a head like a tiny vicious monkey with overlarge red eyes, bare ragged ears, and a set of needle sharp teeth. The monkey spider has a thin body with hair that bristles out giving them a round appearance. The hair on their bodies is poisonous. It is sharp pointed and brittle and will break off in the mouths of predators or in human skin if incautiously handled. All of the monkey spider's paws have an opposable thumb and the first and third sets of paws also have suckers on their fingers that help with gripping to slick surfaces like walls and ceilings. The monkey spider has a disconcerting range of vocalizations most are too high frequency for a human to hear however they also are talented mimics. The monkey spider stays in contact through their vocalizations that sound like half-heard scraps of conversation. When they are angry or afraid they shriek maniacal laughter.
They can move on anything from two to six limbs. They are usually slow creepers except when they are startled by noise or light then they skitter for dark corners and cracks. The monkey spider is despised and feared by all purveyors of magical items especially scrolls because of their diet and reproductive habits.
Monkey spiders lay their eggs in the creases and folds of magical paper, cloth, wood, and leather. Once the eggs hatch, the young eat the magical material. They also like to mark their territory by defecating on metallic magical items. Their feces are highly corrosive and within a short time they can reduce a magic ring to corroded wreck and cause enough damage to a magic sword that the services of a blacksmith and an enchanter are required to repair it. If they hatch within magical clothing, bags, bedrolls, scrolls, or anything near people. The monkey spiders will not come out during periods of activity but will wait until people are sleeping to emerge from their hiding places and begin chewing.
If there are no magical items to eat or mark. The monkey spider will eat non-magical materials and become parasitic blood suckers. The monkey spider is cunning enough to avoid feeding on active victims. Their saliva is a mild anesthetic, and anticoagulant. Monkey spiders especially crave the blood of magic users including druids and clerics, rangers, and paladins. They also love the blood of magical creatures.
Monkey spiders are the bane of flesh golems, piercers, shriekers, and other such dungeon dwelling critters. Almost worse than their destructive habits and blood drinking is the effect their chittering calls have on the dreams of those sleeping nearby. The first signs of a monkey spider infestation is often the deeply disturbed dreams of their unsuspecting hosts.
Besides the damage they pose to magical items and a good night's sleep, the monkey spiders carry disease. Typhus, trench fever, even varieties of the plague are carried by the little critters or their own parasites. Anyone in contact with the creatures must first save vs poison if they do get stuck by their toxic hair then save vs disease. The longer a host is in contact with the creatures the more severe and likely the chance of infection will be. Rumors of infestations spreading leprosy, small pox, typhoid fever, and other magical diseases have been noted.
Note: Their bite is not poisonous and there is magical and pharmacological uses for their saliva so some alchemists or magic users keep monkey spider 'farms' or pay for captured or dead specimens.