A Jaculus is a species first mentioned in Book 1: Keeper of the Lost Cities. When meeting Verdi, a jaculus was on her, and it was described to be a black thing tangled in her feathers. Shown by the fact that Grady was able to hold it in his hand, it is presumably quite small. Its diet consists of blood, and it appears to be a winged serpent. It sucks blood from animals, which shows it is a carnivore, or possibly an omnivore.
In the fifth book, Lodestar, this creature is brought up again, and it is revealed that their venom has a powerful anticoagulant. Its venom is kept at Slurps and Burps, although Livvy decided to go to a gnomish apothecary instead of getting some from Kesler.