I... What?
Okay, look, I know about N being theorized to be a Zorua/Zoroark in disguise, but besides him, I don't think there's any other humans we could speculate to be disguised Pokémon. I do, however, theorize that some Trainers aren't (fully) human, as we know humans to be; they're just humanoids. For the purposes of this thread, maybe I could readapt that theory from "humanoid non-human entities" to "Pokémon we haven't seen before."
For example, I have long speculated that Grimsley is a vampire. Masuda did state that his BW/BW2 design was based off Dracula, after all. Come the Alola games, he's suddenly out of the Elite Four, pretending to be a random traveler, and he looks sort of... anemic. My guess is, he used his gambling activities as a means to procure blood from the losers, until he got caught and had to go lay low somewhere else. (That, or perhaps someone started circulating a portrait of him from back when he was still fully human, prompting the same response.) As a result, he's not getting as much blood as he used to, hence the sudden aging and anemic looks. Oh, and that Fisherman? That's his Renfield, procuring him blood from fish Mons. The point is, Grimsley could be a yet-unseen vampire Mon whose true form is a bat, but has the ability to morph into a human and viceversa - and whose power levels depend on how much blood it has consumed (or how much HP it has).
I also speculate that the Ultra Recon Squad, while visibly humanoid, have a slightly different organic structure than humans from the "main" Pokéverse as a result of evolution. Who knows, they might even share some features/organs in common with Ultra Beasts, seeing as how main Pokéverse humans might share a common origin with regular Pokémon from their own universe!
