I would like to suggest a Nextbot for John Carpenters The Thing,

How it can work:
The Thing can infect humans and combine and then begin to assimilate them. Once The Thing has assimilated a human it will stay near its teammates or the player and wait for its moment to transform which will partially have the original appearance of the human/combine/player skin but more deformed and body-horror like. While The Thing is in its true form it can insta-kill humans/players by grabbing them for assimilation or vomiting on them as an assimilation projectile attack. If The Thing is isolated from the crowd with another human it will preform a sneak attack like AmongUs where it grabs them in its human form and then briefly transforms to assimilate them without leaving a death message.

Special Ability's:
As mentioned before when The Thing is in its true form it can preform an instant-kill grab attack or while its identity is concealed it can preform this attack more sneakily when it is alone with another human. It will also have a vomit attack in its true form which will assimilate anyone who has been contaminated by it.

How Assimilation Works:
When a human/player/combine is assimilated they will have their original skin but their AI will make them try to blend in with the others, if its an enemy NPC like combine it will still attack players/humans but it will try to assimilate other combines, if a human is infected it will blend with other humans and can be commanded to follow the player like usual however it will try to assimilate them. If a player is assimilated it'll retain the original skin even if its from another modpack similar to the custom zombie skins mod and it'll mimic the behavior of a player just like the Lambda Players mod, additionally when a player is assimilated it can use chat with the original players username and try to lure other players in, it will also command a low number of human NPCs to follow it to isolated locations so it can assimilate them. If an assimilated player encounters the respawned player that it just assimilated it will accuse them of being an impostor in chat and attack them. When an assimilated host is attacked it will react like normal however once it is killed in human form it will reveal its true form.

Counter:
Players can use Molotov and flamethrowers against assimilated hosts, these will immediately force The Thing to reveal its appearance before it has died and kill The Thing very quickly. Additionally there will be a bloodtest kit that allows the player to poke NPCs for their blood or scrape a sample off a blood decal and then put the blood in the petri dish, they can stick the heat prong in the petri dish and see how it reacts, if it does nothing they are innocent however if it jumps out of the petri dish it will expose The Things host. If the bloodtests positive then that host for The Thing will immediately give up and reveal their true form.