In the past, studies on laterality of auditory processing focused mainly on primates and birds. Communicative signals' species-specificity has been largely investigated as a factor susceptible to influence hemispheric specialization, but little is known concerning the impact of social familiarity. Here, we tested the influence of the degree of emitter-receiver familiarity on auditory laterality in domestic horses, using the widely used head orientation behavioural paradigm. As the species shows visible reactions to sound, with one or two ears moving towards a sound source, we integrated ear orientation as a further behavioural variable. We could therefore dissociate visual implication, usually integrated on the traditional measure of head orientation, from auditory laterality. By comparing responses to the playbacks of different conspecific whinnies (group member/familiar neighbour/stranger), we demonstrated a left hemisphere preference for neighbour calls while no preference was found for group member or stranger calls. An opposite pattern of ear side preference appeared for neighbour vs stranger calls. To our knowledge, these results are the first to demonstrate auditory laterality in an ungulate species. They support the idea that not only a call's social value, but also the degree of attention of the listener could influence auditory laterality.