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I feel like a human-mimicking insect monster as a gym leader would break the setting too much. Working within parameters, maybe you could have a Pheromosa relative species posing as a human (poorly, like maybe just wearing a hat or tattered cloth cloak if there needs to be visual difference from common ones) and just directs other bug-type Pokémon in battle.
Location is abandoned subway tunnels underneath the city, copying the movie Mimic blatantly. The gym trainers are lost humans who don't know what is going on and leave if you defeat them, only saying they fought some powerful trainer they didn't get a good look at. The gym battle room itself looks like a nest, although the building materials would vary depending on what the actual motif species is (presumably not a cockroach to make it distinct from Pheromosa). Would approach by turning around from a sitting position, walking up strangely, making a cry, and then the battle against Gym Leader(?) *Name blatantly derived from species name*. For species that would make sense for this environment, Nincada, Wormadam Trash Cloak, Durant (has Truant and really bad movepool), and something new. Since throwing something with or so 570 BST would be unfair, the "Gym Leader" just flees, leaving behind something shiny objects that seem to be a badge and a TM for Bug Buzz or something. The image after being defeated in the badge case is a blurry photo that lacks the detail to be understood as not-human, and quote is nonsense like "Guh-krrek klik!"
When you rematch in the post game, the different environment means a very different team. Maybe Vespiquen, Galvantula, Volcarona, Scizor, new species' evolved form , and the leader itself. Despite the leader's species being seen elsewhere as capturable, no one notices not being human.
