During bursty MHD events, transient ion cyclotron emission (ICE) is observed from deuterium plasmas in the Large Helical Device (LHD) heliotron-stellarator. Unusually, the frequencies of the successive ICE spectral peaks are not close to integer multiples of the local cyclotron frequency of an energetic ion population in the likely emitting region. We show that this ICE is probably driven by a subset of the fusion-born protons near their birth energy E_p = 3.02MeV; this subset has a kinetic energy component parallel to the magnetic field, m_pv_para^2/2, signicantly greater than its perpendicular energy m_pv_perp^2/2, for which v_perp ~ V_A, the Alfvén speed. First principles computations of the collective relaxation of this proton population, within a majority thermal deuterium plasma, are carried out using a particle-in-cell approach. This captures the full gyro-orbit kinetics of all ions which, together with an electron fluid, evolve self-consistently with the electric and magnetic fields under the Maxwell-Lorentz equations. We find substantial frequency shifts in the peaks of the simulated ICE spectra, and these correspond closely to the measured ICE spectra. The present study proposes that the transient ICE in LHD is generated by the identified subset of the fusion-born protons, relaxing under the magnetoacoustic cyclotron instability. So far as is known, this is the first report of a collective radiation signal from fusion-born ions in a non-tokamak magnetically confined plasma.