A jaculus is a species first mentioned in the original book. When meeting Verdi, one such of these animals was on her, and it was described to be a black thing tangled in its feathers. Shown by the fact that Grady was able to hold in his hand, it is presumably quite small. It's diet consists of blood, and it appears to be a winged serpent. It sucks blood from animal, hence showing it is a carnivore or possibly omnivore.
In the fifth book, Lodestar, this creature is brought up again, and it is revealed that their venom has a powerful anticoagilant. It's venom is sold at Slurps and Burps, although Livvy decided to go to a gnomish apothecary.