The Eaglefly is a dragon mentioned on the official Website for the How to Train Your Dragon Books.
There is no information or images of this dragon's appearance. Based solely on this species' name, the Eaglefly may look like an eagle in appearance and/or size. Because other hunting dragon species are smaller in size, we might infer that an Eaglefly is also too small to ride, and perhaps in the size range of dogs.
Because the Eaglefly is listed as the species of hunting dragon that both Tuffnut Junior and Speedifist have, we know this dragon is trainable. They might be relatively easy to train, as both Tuffnut Junior and Speedifist are Viking Novices.