  Strong coupling between magnon and electromagnetic wave can lead to the
formation of a coupled spinphoton quasiparticle named as magnon-polariton. The
phenomenon is well studied for ferromagnetic systems inside microwave cavities
in recent years. However, formation of magnon-polariton is rarely seen for an
antiferromagnet (AFM) because the strong coupling condition is not easily
fulfilled. Here we present time-domain terahertz measurement on a multiferroic
polar antiferromagnet Fe2Mo3O8. We find clearly beating between two modes at
frequencies above and below the electric-active magnon frequency below TN,
which we assign to the formation of AFM magnon-polariton. An ultra-strong
spin-photon coupling effect is derived based on the energy level splitting.
However, the AFM magnon-polariton is absent in the frequency domain
measurement. Our work reveals that the coherent magnon formation driven by the
ultrashort THz pulse provides a new way to detect polariton mode splitting.
