Green Dragon is found in his cave on Crandor Isle.
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15000 exp to Defence and Strenght and right to buy and wear Rune plate, It also gave access to crandor isle through the strange looking wall (hiden exit) near the red spider respawn inside karamja volcano.
First dragon was introduced with a Dragon Slayer quest et end of september 2001. It was one Green, fire breathing dragon called Elvarg. His fire can roast you for over 60 Damage making any fight very short IF you are without antidragon breath shield. As it is quest monster you can kill it only once and after that you are teleported out of cave.