McCool was a Starfleet shuttlecraft, registry number NCC-1764/6, attached to the USS Defiant in the 2260s. The McCool disappeared along with the Defiant into an interphase rift in Tholian space in 2268.

After the Defiant was recovered in a parallel universe by the Terran Empire, the McCool was stolen by Commander T'Pol and turned over to rebel forces on Vulcan. (ENT - Mirror Universe novel: Age of the Empress)

Presumably named for astronaut William C. McCool, pilot of the Space Shuttle Columbia when it disintegrated during atmospheric re-entry in 2003.