I think it is.
with the actual Damage-dealing Poisons being released in U13, I see NO REASON for the devs not to give us a Green Dragon.
Make it an Ultra-Rare (1:100 chance to spawn) Mob in the Kings Forest, or maybe the boss of its own side Quest in there.
(cool idea for breath weapon copied from Sirgog)
Make its Breath have a lingering effect, similar to how Cloud spells work.
Just a thought