Antineutron is the antiparticle of a neutron. Anti-neutrons were found in 1956 at the University of California, Berkeley. They have the same mass and net charge (0) as the neutrons. The only difference is that they have the opposite baryon number, that is, the quarks inside the neutrons have opposite spins to the anti-quarks in the anti-neutrons.
Anti-neutrons annihilate on contact with their antiparticle, neutrons.