She doesn't react at first when the monster appears. There's no one else in the landing bay for her to check the senses of, after all, and River knows by now that she sees things differently. A giant snake bearing a mirror really isn't all that strange. She tilts her head, examining it, wondering if her hallucinations are trying to tell her something or if she needs more sleep. Probably she needs more sleep.
(Of course she's hallucinating it has a mind, too, but, well, snakes don't just appear, and it's not like she can't imagine simple animal minds - ).
It's moving towards her. River wonders if it'll vanish when it touches her. Touch is more iffy, in hallucinations.
And then she passes through the mirror -
And she's not in Kansas anymore. (No, it wasn't Kansas that was Through the Looking Glass, that was... The Queen with the pawn promotion metaphor? She can't remember where Alice was from... River didn't bother memorizing a lot of geography for Earth That Was.)
And then River notices she can feel quite a lot of intelligent minds, a city a lot, and she usually can't hallucinate in this detail the flow of everyone -
She freezes, eyes darting around, focusing on who's nearby, on being receptive to flickers of danger -