Aww thanks, Olli. :3 I wouldn't mind a Dragon/Poison Pokemon being a legendary Pokemon either, as long as we have a Dragon/Poison Pokemon, I'd be happy.
When I said this earlier:
"Anyway Hooktail's, poison-based attacks are fairly strong. His Megabreath is the third strongest move in the game, according to the Super Mario Wiki."
I was meant to write Gloomtail there, not Hooktail. >.<
Another idea: perhaps a stingray Pokemon, so Water/Poison. We only have Tentacool and Tentacruel with that type combination, so I'm sure there is a lot of potential to develop another Water/Poison Pokemon. There is only one mantaray originated Pokemon too, being Mantine which is Water/Flying, with no Poison-type traits at all. :)