Why do you think there hasn't been a legendary Poison type Pokémon yet? What do you think one would look like? What role in the universe do you see it having?
Legendary Pokémon tend to be Pokémon equivalents of real-life myths and legends, I could understand that a poison-based myth could be somewhat difficult to come by. However, considering that there are now birds of death, legendaries involved with the environment, and quite a few references to alchemy, I can see something involving poison being more than possible.
Though I know that Ultra Beasts aren't viewed as legendaries, I believe that Nihilego is a pretty good idea for what a poison legendary could be like. Or it could be something similar to being the Cthulhu of the Pokémon world. That'd be pretty cool. A Pokémon that seeks to control (this idea could be a Psychic/Poison type, which would give it a unique typing as well). I could also see it representing decay or entropy, which would be pretty cool to see as well.
Though I know that Ultra Beasts aren't viewed as legendaries, I believe that Nihilego is a pretty good idea for what a poison legendary could be like. Or it could be something similar to being the Cthulhu of the Pokémon world. That'd be pretty cool. A Pokémon that seeks to control (this idea could be a Psychic/Poison type, which would give it a unique typing as well). I could also see it representing decay or entropy, which would be pretty cool to see as well.