Balok's pilot vessel was the small craft that separated from the enormous Fesarius in 2266 to tow the Enterprise away, and aboard which Kirk finally met the real Balok.
The pilot vessel appears in the original series episode The Corbomite Maneuver. After Kirk's bluff about a corbomite device stops Balok from destroying the Enterprise, the Fesarius releases a far smaller ship, which locks on a tractor beam and begins towing the Enterprise to a planet where its crew are to be interned.
Kirk strains his engines to breaking point to tear free, and the effort leaves the little ship disabled and sending a weak distress call. Kirk decides to answer it, and beams over with McCoy and Lieutenant Bailey.
They find that the fearsome face on the viewscreen was a puppet. Balok is small, childlike and hospitable, he serves them tranya, and he explains that he was testing their intentions. Bailey stays behind as an envoy.