Tapu seem to be 70 base health, then 130/115/95/85/75 in various other stats. Koko has 130 in Speed, Lele in Sp. Atk, Bulu in Atk, and Fini in Sp. Def, leaving Def. They also each have a Terrain-starting ability, which currently cover Electric, Grass, Fairy, and Psychic. Weather effects to "boost" Fire, Water, and Rock, with Ice, Ground, and Steel ignoring hazardous weather, and Flying having Delta Stream.
This leaves Fighting, Poison, Bug, Ghost, Dragon, and Dark for Types that could have a Terrain added, before thinking about what makes *sense* to have a Terrain or Tapu. Fighting and Bug have the least sense for a Terrain, Dragon's a bit "high" for a Tapu, Ghost and Dark is just begging for repeating the *usual* exile notion, while Poison would fit with a hazardous terrain *and* screwing with the other Tapu without actually being antagonistic.
Poison/Fairy has the additional upside of being weak to only Ground, Psychic, and Steel, with two of those being predominantly physical for the 130 Def making it not be outright glass to them, leaving *just* Psychic as a "full" vulnerability.
Could be more referential to Native American, African, or Voodoo aesthetics than the Pacific Islander visual queue of the other Tapu, furthering the visible friction with the other Tapu but still being close enough to be *recognized* as one.
Rather than pure stall, I'd look to tuning it for Nature's Madness leading to Poison percentage-damage clearing at a decent pace. Not a sweeper, not a wall, but a mid-range time-to-respond Pokemon.
