What subatomic particle has the same mass as a proton?

No two subatomic particles have the exact same mass. The proton and the neutron, however, come close. Neutrons are heavier than protons by only 2.3 X 10-30 kg. (Neutron: 1.674927351 x 10-27, Proton: 1.672621777 x 10-27)