An electromagnon in the magnetoelectric (ME) hexaferrite Ba 0.5 Sr 2.5 Co 2 Fe 24 O 41 (Co 2 Z-type) single crystal is identified by time-domain terahertz (THz) spectroscopy. The associated THz resonance is active on electric field (E ω ) of the THz light parallel to the c axis (|| [001]), whose spectral weight develops at a markedly high temperature, coinciding with a transverse conical magnetic order below 410 K. The resonance frequency of 1.03 THz at 20 K changes -8.7 % and +5.8 % under

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... nal magnetic field (H) of 2 kOe along [001] and [120], respectively. A model Hamiltonian describing the conical magnetic order elucidates that the dynamical ME effect arises from anti-phase motion of spins which are coupled with modulating electric dipoles through exchange striction mechanism. Moreover, the calculated frequency shift points to the key role of Dzyaloshinskii-Moriya interaction that is altered by static electric polarization change under different H.