Giant spiders are a mainstay of fantasy, all the way back to Shelob and Mirkwood...
Jewel-jaws are pretty blue-winged carrion-eating butterflies. (Stolen from P C Hodgell.)
Bladeflies are butterflies with iridescent, crystalline razor-edged wings which drink blood instead of nectar. They normally attack small animals, but when they run low on food they congregate in large swarms and go after larger creatures. Their glittering wings have a hypnotic effect in good lighting. They are repelled by fire and extremely pungent smells such as garlic. (Stolen and adapted from the Net Monstrous Compendium.) The wings are fragile, but make for beautiful decorations, and some wizards have captured many large swarms for the purpose of producing enchanted weapons, clothing and armor. (At least one enchanted sword made from fused bladefly wings is extant in the world.)
Skyjellies are jellyfish that can pump up a gas bag full of light, fiery gas and ascend into the sky; they vary from the size of a man’s head to a small hut. A sail-like membrane on top of the skyjelly allows it to catch the wind at various angles and travel where it wishes, usually toward water. Their usual mode of hunting is to drift through the air with their tentacles dangling in the water, but they are not averse to drifting over open terrain in the same manner. The tentacles, while fine, are capable of retracting up into the skyjelly, taking prey with it to be dissolved in the creature’s digestive juices. Farmers have been known to lose livestock to these creatures, and archers are much in demands when the skyjellies are spawning.
Skyjellies explode whenever a hole is made into their gas bladder and a spark catches the flammable gases within. They reproduce in water, and seem to be able to handle both fresh and salt seas. They travel in pods.