"It's not something local to the area! Nothing in the forest would be so upset about something native. Or even something with such a long-standing colony like the acromantulas. It might be a dragon. Every once in a while some wizard gets it in their head to try and raise one and has to release it shortly after. Or a chimera. Or--"

And as they continue walking he gives a run down of all the dark or dangerous creatures it could possibly be based on the evidence they have. None of them are pretty and some might well be suicidal to face even with two well-trained wizards. If nothing else, though, it's obvious that he knows in great detail what he's talking about.