There are so many.
Like the Pluto from Modern Sounds ( thread )
One could look through various other threads around here with rich sources for this, like the Crowdfund thread, or the Mysterious Instruments thread, or the Unique Dirty Raw Fun Synths thread.
How about the Sismo ICE ? ( post )
I think it is probably good to be explicit that this isn’t about Roland Boutique, as they seem to dominate that word association.
How about the Ellitone E[64], which is rare and unusual ? ( video )