So, in ESO, Werewolves take more damage from poison. Similar to how Vampires take extra fire damage (except Vamp is based on stages, Lycans only when turned). However, I've never noticed this in TES before. Why would Hircines hunters, take more damage from something associated with hunting? Why isn't it just silver, that makes sense.

So has a weakness to poison been a thing for Lycans in the past, or is this just something new in ESO?

Gameplay unique to ESO. Ordinarily, Lycanthropes got a boost in Endurance, which actually rendered them more resistant to things like poisons and that nature (And given the increase in size, this makes sense). This to me sounds more like a case of balance in gameplay coming first.