Well, I like Doctor Who as some people may know. Why not something like the Weeping Angels? (If you don't know what they are, look them up). They could be a very rare occurrence, on only certain planets, but a kind of enemy that you didn't want to turn your back on, for fear of something terrifying happening.
Well, I thought about that being a problem. I was thinking they would move if you turn away, and the blinking part would be a bit more difficult, maybe if you swapped tools?
That would just be god dang freaky . Having to, you know, look at my back every second for it (Even if it's a rare occurrence, holy dang it's just freakin scary if it does pop up and just stressful to look for it every second!)
or this could be a playable racist, that an angels could be friendly or foe (stories about the fallen angels) and see how u react to the behaviors
Perhaps only wearing certain headgear, entering attack range, or turning toward the opposite direction of the weeping angel while it has line of site would trigger aggression. If the AI wasn't too difficult to code/program this kind of monster could be pretty intimidating, especially at lower levels. Anyway great idea.