The Green Beasts were created sometime before the Shadow Lord's first invasion of Deltora. Exactly where they fit into the Shadow Army is unclear, but occasionally some would escape from the Shadowlands and prowl the Dread Mountain area.
Tales of Deltora
A group of Dread Gnomes were hunting a Green Beast when Adin came to Dread Mountain to ask for the Emerald for the Belt of Deltora. The Gnomes captured him and intended to use him as bait for the beast, but their trap failed and the beast nearly killed their leader — Az-Zure —before Adin was able to slay it.
The Kin whom Lief, Barda, and Jasmine met never mentioned the Green Beasts when they spoke about the dangers of Dread Mountain, so it is possible the Green Beasts had gone extinct by this time or at least on Dread Mountain.
Green Beasts are large, bipedal green-scaled lizards with three-clawed hands and orange eyes. The scales on their bodies are thick enough to repel weapons, but their throat-scales are less durable and can be pierced by a spear, sword, or heavy arrow.
Green Beasts are highly aggressive and will attack nearly anything that they encounter, but they are not particularly intelligent.
Green Beasts are carnivorous and will eat most living creatures.
Green Beasts roamed through the Shadowlands, though occasionally some would venture into the area around Dread Mountain.
Green Beasts could be seen as a prototype to the Vraal, since both are humanoid green reptiles created by the Shadow Lord.