"I'm manipulating him!" "Nope, it's a monster!"

This thought came into my mind a minute ago: what happens if a PC is interacting with a monster he doesn't know is a monster and the player wants to manipulate it? As the MC, should I say "You can't, it's a monster!" (or maybe "Man, there's no way: this f***er just doesn't seem to respond to your body language!")?

I would go for the second option, of the two.

In general, make sure that you get to whats's happening in the game. So, if the player says "I'm manipulating him!" then my first response is something like "Okay, so what do you say to him? Do you just ask him to do it?"

By playing out the conversation you get the opportunity to show that a manipulate attempt won't work without having to go into things the hunters haven't worked out. When it gets to the critical moment, "I need you to let us into the factory after hours," say, then you can have the response be more natural. It could be "well, you can tell this guy isn't going to budge on this." or "you get the feeling there's nothing you can offer that will make him consider it" or even "there's just something wrong about him, like he's sizing you up to eat you instead of listening".

Note that this also works for perfectly normal people who the hunters are asking to do something really unreasonable!

Thanks!