The leak is weird, but in my experience true leaks tend to be weird. The Affleck leak, for example, mentioned Sirfetch’d, but no Galarian Farfetch’d. And there was a confirmed XY leak that mentioned Mewtwo had two new formes (when we only knew about one) activated by items, but never mentioned the words “Mega Evolution”. Mega Evolution was so game changing that you would think anyone who played XY early would have leaked it, and yet that leaker didn’t.

Anyway, this Isle of Armor leak claims there’s a Poison Eeveelution. I think it’s possible. If you look at the leaked items, two of them are Galarica Cuff and Galarica Twig.
Galarica Cuff is likely the item for Slowbro. The lore is likely “Slowpoke wears the Cuff, it attracts a Shellder, in trying to bite the Cuff Shellder causes Slowpoke to evolve”.

If that’s the case, then what about the Galarica Twig? It could be another evolution item, but we know Slowking is not coming until Crown Tundra, so that can’t be it. Maybe it’s the item for Eevee?

Quick prediction: Poison Eevee will be mushroom-themed. Many mushrooms are toxic, and if the Twig is the item, many mushrooms grow on trees. It might also have a snake tongue.

By the way, someone mentioned on Reddit that Galarica seeds could be a reference to Brassica seeds (mustard seeds), and that “mustard gas” could be why Slowbro is Poison-type. I’m not sure if the Japanese name also references mustards in anyway, but Slowpoke’s design seems to reference mustard. We even have a character named Mustard.
If that reference theory is true, it makes sense both of the Galarica items are for Pokémon that become Poison-type.