I'm doing a Low Level Game Challenge and a Rage Completion Challenge at the same time now, and I'm totally sure I encountered Over-Mind on the Phantom Train, because I worked VERY thoroughly with this stuff (YAY Djibriel and his Rage Guide!)
However, I've been running around on The Veldt for three hours now, and I've encountered every single enemy lots of times (among them, Telstar x 4), EXCEPT for the Over-Mind.
Is he that rare, or did I miss him after all?