Well, we can rule out Praeses routinely having the forms of large humanoid shadows with glowing eyes, otherwise Gavia and Ardent wouldn't have been so scared.
I do think it likely Alice is messing around with them to teach them not to sneak out without her. But if this thing is totally independent of Alice ( then I'm not completely sure it's intentions are bad, as both kids seemed to think. Maybe it was trying to say hello, or was just curious. Or maybe it was trying to protect those silly young idiots from the REAL dangers of being out at night in this bizarre place, which they are about to run into in a panic. But if it's intentions are bad, then Alice is about to do what she does best: rescue helpless idiots.