I have a limited understanding of antiparticles, so this may be why I am unable to explain why certain mesons are their own antiparticles, while others are not. My understanding is that antiparticles have properties (ie. spin, charge, mass, etc) which are of the same magnitude, but opposite signs, of their elementary particle.
My question is, why is the charmed eta meson its own antiparticle, while the neutral kaon is not its own antiparticle?