devtrackers.gg
Just noticed the Haunted Cave milestone has silhouettes of a shellback, grabber, and praetorian in addition to the bulk detonator. I guess these other ghosts were deemed too annoying, lethal, weak, or otherwise inappropriate for the vibe GSG was going for with Haunted Caves?
I'm not aware of variations of the Ghost ever being planned. It might be more of an artist rendition of the concept than anything else.