An antineutron shares similar properties to that of a neutron, however its spin directly opposes that of a normal neutron.
In Spock's memory test on Vulcan in 2286, one of the questions was: "Adjust the sine wave of this magnetic envelope so that antineutrons can pass through it but anti-gravitons cannot." (Star Trek IV: The Voyage Home)
In 2375, Tuvok detected antineutrons in the remains of the Silver Blood USS Voyager. (VOY: "Course: Oblivion")