I believe that's the only actual unused sprite in SC.
However:

Cargo ship, mercenary gunship, raider, repair bay, and that one doodad on the jungle tileset (you can't select it).
Worth noting, Arcturus Mengsk is a Battlecruiser unique unit (1000 HP, 250 MP, cannot use Yamato Gun).
I'll have to look through my copy of the beta again. It has actual sprites for the cargo ship and merc gunship, don't remember if it has one for the raider, cantina, etc.
EDIT: Unused portrait (nbiker):

It does not animate.
EDIT: Unused unit names (from the beta):
Merc Biker<0>*<0>Independent Units<0>Normal<0>
Badlands Critter<0>*<0>Critters<0>Normal<0>
Space Critter<0>*<0>Critters<0>Normal<0>
Cargo Ship<0>*<0>Independent Units<0>Normal<0>
Merc Gunship<0>*<0>Independent Units<0>Normal<0>
Raider<0>*<0>Independent Units<0>Normal<0>
Grom<0>Biker<0>Independent Heroes<0>Normal<0>
Spot<0>Badlands Critter<0>Critters<0>Normal<0>
V'ger<0>Space Critter<0>Critters<0>Normal<0>
Sally<0>Cargo Ship<0>Independent Heroes<0>Normal<0>
Greedo<0>Gunship<0>Independent Heroes<0>Normal<0>
Boskk<0>Raider<0>Independent Heroes<0>Normal<0>
Peter<0>Raider<0>Independent Heroes<0>Normal<0>
In SC (vanilla), Badlands Critter is replaced with Rhynadon (Badlands Critter), Space Critter was replaced with Bengalaas (Jungle Critter), Spot was replaced with Ragnasaur (Ashworld Critter), and V'ger was replaced with Critter (Space Critter) (still unused). Brood War used most of those unit slots for some of its units, so they are not present anymore in that version.
Cave<0>*<0>Neutral<0>Unused<0>
Cave-in<0>*<0>Neutral<0>Unused<0>
Cantina<0>*<0>Neutral<0>Unused<0>
Mining Platform<0>*<0>Neutral<0>Unused<0>
Independent Command Center<0>*<0>Independent<0>Unused<0>
Independent Starport<0>*<0>Independent<0>Unused<0>
Jump Gate<0>*<0>Neutral<0>Unused<0>
Ruins<0>*<0>Neutral<0>Unused<0>
Kyadarin Crystal Formation<0>*<0>Neutral<0>Unused<0>
Neutral buildings. There is a Zerg ability called "Infest Mine", presumably for use with the Mining Platforms.
Lost unit graphics:

Cantina, Raider ("gunship.grp"), Merc Gunship ("raider.grp"), and Cargo Ship.
There are also damaged versions of Terran buildings (too many to post right now), so the weirdest one:

This is... a bunker. I don't get it. |