Latin name: Jaculus
Other names: Iaculus, Javelin-snake, Javelot
A flying serpent

General Attributes
The jaculus is a serpent that can fly. It hides in a tree until an animal passes underneath, and then it throws itself down on the animal and kills it.

Sources (chronological order)
Lucan [1st century CE] (Pharsalia, book 9, verse 848): “Swift Jaculus there…”. (verse 962-966): “Upon branchless trunk a serpent, named / By Libyans Jaculus, rose in coils to dart / His venom from afar. Through Paullus’ brain / It rushed, nor stayed; for in the wound itself / Was death…”.

Pliny the Elder [1st century CE] (Natural History, Book 8, 35): The iaculus hurls itself from the branches of a tree, so that it is not only dangerous to the feet, but flies through the air like a missle from a catapult.

Isidore of Seville [7th century CE] (Etymologies, Book 12, 4:29): The iaculus is a flying snake. They jump from trees and dart onto passing animals, from which they get their name, darter (iaculi).

Source: Medieval Bestiary.

all the animals of Satan’s bestiary, assembled in a consistory and set as guard and crown of the throne that faced them, singing its glory in their defeat, fauns, beings of double sex, brutes with six-fingered hands, sirens, hippocentaurs, gorgons, harpies, incubi, dragopods, minotaurs, lynxes, pards, chimeras, cynophales who darted fire from their nostrils, crocodiles, polycaudate, hairy serpents, salamanders, horned vipers, tortoises, snakes, two-headed creatures whose backs were armed with teeth, hyenas, otters, crows, hydrophora with sawtooth horns, frogs, gryphons, monkeys, dog-heads, leucrota, manticores, vultures, paranders, weasels, dragons, hoopoes, owls, basilisks, hypnales, presters, spectafici, scorpions, saurians, whales, scitales, amphisbenae, iaculi, dipsases, green lizards, pilot fish, octopi, morays, and sea turtles.

Source: The Name of the Rose by Umberto Eco.