Well my vote would be clearly for the Mage Tech worlds. before we take on the AI planet.
And Mind Flayers! one of the Elder races? who have one of the largest sources of knowledge and know one has heard of them?
I would expect they at least one would be part of the Explorers Guild,unless no one but the Captain has ever been to your world..or that they don't exist on any other planet...
Thirdly if whatever the Captain did with the creature,if that involved Magic, would it have been lesson training that Kartesh could have sat in on and observed,or the Captain could've explained on completion (if he observed and didn't interupt the Captains work)
and what info came up about the Yautja...?