Sceptrigon
Armored Legend
This.
Though, it'll probably be an event if there aren't any side legendaries introduced outside of the main trio again
Sun and Moon don't sound like something that'd be Poison.
I've been hoping for a Poison type legend as a mascot for a while, and I think I thought of something interesting. I was wondering about Sun and Moon being a new gen still closely connected to Kalos, and what Game Freak could be planning next. What if they reveal the next set of games after Sun and Moon to feature Zygarde complete and 7th gen's third mascot legendary as a pair? Since Zygarde represents order, the other legend could be based off of chaos, and Poison might be fitting for it. Basically just something I thought about and how Zygarde could be left out purposely in order to be featured in the next games after Sun and Moon.