For Pex, Water/Poison already is a pretty good typing, so you don't need to change it. It also allows Pex to absorb Toxic Spikes, which can be a huge advantage in hazard stack battles.
If you want it to check Zygarde and other ground-types, Water/Bug (with Infestation) is another option, but you give up a moveslot, as Infestation is not really useful compared to its other options.
Since USUM gave it Knock Off, it can now also mimic Alolan Muk's Dark/Poison typing to become a check to most psychic types while still being able to absorb Toxic Spikes.
Oh, and I would really appreciate a Kyurem-Black/Kartana suspect.
For stall teams, after a Kyurem-B ban, the regular Kyurem with Flying/Steel might be an alternative with slightly less offensive presence but similar bulk.
If you want it to check Zygarde and other ground-types, Water/Bug (with Infestation) is another option, but you give up a moveslot, as Infestation is not really useful compared to its other options.
Since USUM gave it Knock Off, it can now also mimic Alolan Muk's Dark/Poison typing to become a check to most psychic types while still being able to absorb Toxic Spikes.
Oh, and I would really appreciate a Kyurem-Black/Kartana suspect.
For stall teams, after a Kyurem-B ban, the regular Kyurem with Flying/Steel might be an alternative with slightly less offensive presence but similar bulk.