the mutant guy

xeno doesn't appear until the end. your theory just sounds wrong. they probably threw in that clip just to make you think that. it's something that changes him into a violent big headed mutant. acid doesn't do that. some type of parasite probably.

we think xeno might appear at the end! but yeah, something happens to him, possibly turning him into our psychotic friend at the top of the thread.

There's no way to be sure of anything from what we've seen so there really isn't any point in speculating. All I can tell from what I see is a guy with makeup on his face is jumping off of something at someone while in a wire harness. Maybe he's freaking out because his suit depressurized? Maybe he's been infected with something that alters his mind? Maybe he's a doppleganger sleeper cell alien (like a shapeshifter) that was pretending to be one of the crew? Maybe he's just a dumb guy with makeup on his forehead? Whatever he is, I doubt he's the main villain or creature. We don't know and we can't tell right now.

He's a crew member who has been infected by alien dna.