Fly was born sometime after Thaegan's return to Deltora. His father, if there was one, is unknown. Fly and his twelve siblings roamed Deltora's northeast, terrorising the countryside.
The Deltora Book of Monsters
Fly and his siblings captured a group of travellers and ate all but one before falling asleep. The man was able to escape, though he had no memory of anything before the incident. Josef met this man, and learned from him information about Thaegan's children for his book. 
Fly, Fie, and their nine surviving siblings—Hot, Tot, Zan, Zod, Pik, Snik, Lun, Lod, and Ichabod—join forces to seek vengeance on Lief, Barda, and Jasmine for the deaths of their mother ad siblings. As wolves, they pursued the companions and caught them in a net trap. Lief tricked them into fighting one another over how to equally divide the three people among eleven monsters. The fighting became lethal, and Fly and all his siblings died, except for Ichabod. 
In the anime he and his twin Fie are accidentally killed by their mother when one of her killing spells went haywire.
Fly was a green monster with a large head and dripping brown fangs.
In the anime adaptation, Fly is a green monster who looks much like his brother Fie. Fly has yellow eyes, while Fie has red eyes - which is the only differentiation between the two.
Like all his siblings, Fly had shapeshifting powers that allowed him to take on the form of a wolf, and possibly other forms as well.