This class of vessel was in use during the 2260s.
In 2266, the USS Enterprise rendezvoused with the Antares. The Antares was later destroyed when its previously warped baffle plate was removed by Charlie Evans, the ship's former passenger, who claimed the part was already defective and already bound to fail. (TOS-R: "Charlie X")
By 2268, this class was considered to be "old-style", had been converted to automation. That year, the Enterprise, under the control of the M-5 multitronic unit, destroyed the unescorted Woden that was hauling ore near Alpha Carinae II. (TOS-R: "The Ultimate Computer")
In 2269, the Enterprise was assigned to escort two robot grain ships carrying a load of quintotriticale to the Sherman's Planet to assist in the survival of the colonists who were experiencing crop loss and famine.
During an encounter with the IKS Gr'oth under the command of Captain Koloth, one of the ships was disabled, and its cargo was transferred to the Enterprise. The Gr'oth later returned and attacked the second ship, disabling its propulsion systems as well. The ship was later recovered and towed by the Enterprise to Sherman's Planet. (TAS: "More Tribbles, More Troubles")
The Antares-type was fairly complicated in overall design. It was constructed on a horizontal spaceframe with the standard two-nacelle configuration. Jutting above and below the main spaceframe were the ship's cargo holds, with the largest being located below the axis of the vessel. The forward living section for a standard crew of twenty and bridge were located in the foremost position of the vessel. The deflector dish was centered in between the bottom of the forward living section and the bottom of the ship (TOS-R: "Charlie X")
The Antares-type was smaller in size than a Constitution-class starship, however, overall it possessed much larger cargo holds than were found in the latter design. (TOS-R: "Charlie X"; TAS: "More Tribbles, More Troubles")
Antares-type ships were easily convertable to automation, with the removal of the forward bridge section. These automated vessels were essentially flying cargo holds. (TOS-R: "The Ultimate Computer"; TAS: "More Tribbles, More Troubles")
Ships of the class
- TAS: "More Tribbles, More Troubles"
- LD: "Terminal Provocations"
The Antares type originally existed as the robot grain ship, which originally appeared in Star Trek: The Animated Series. This class was the second Federation starship design, at the time, to appear in the Star Trek franchise.
The final draft script of "More Tribbles, More Troubles" (dated 23 April 1973) described these "robot grain ships" as "two identical crop carrying robot ships, not as big as the Enterprise, but every bit as powerful. Because they have no life support modules, they can use all their power for payload."
The design was adapted for the remastered version of Star Trek: The Original Series because Michael Okuda highly approved of the configuration. He liked how it wasn't, like some of the other TAS vessels, too impressionistic. "It was obviously well thought out and it had a wonderful quality that you could quite clearly see came from Matt Jefferies' universe," recalled Okuda. "It seemed to follow the same design principles as the Enterprise, and yet it was dramatically different. It was my favorite ship on The Animated Series." The original animated design heavily inspired the CG model of the Antares, the Yorkshire, and the Woden, which appear in the remastered TOS episodes "Charlie X", "Court Martial", and "The Ultimate Computer" respectively. The same CG model was used for the Yorkshire and the Woden because everyone involved in depicting the Antares on screen was pleased with how the appearance of that ship turned out. (Star Trek: The Official Starships Collection, issue 63, pp. 12 & 13)
The Antares and the Yorkshire had a bridge and living section amended to the body of the original design. The front extension as a crew module was confirmed by Producer Dave Rossi in an interview. 
According to the Star Trek Encyclopedia (4th ed., vol. 2, p. 326), the starship class of the Antares in "Charlie X" was called Antares-class. The USS Hermes, on p. 330, was listed as a sister ship of the Antares.
According to Doug Drexler, on notes on his Facebook page, the Antares-type freighter was an older class of ship than the USS Enterprise. These ships measured 470 feet in length with the crew module, and 343 feet in length without it. For the Antares and the Yorkshire, the name was on the side of the crew module, whereas, for the Woden, the name was on the side of the main hull.