Behavioural synchrony among individuals is essential for group-living organisms to maintain their cohesiveness necessary to get information and against predation. It is still largely unknown how synchronization functions in a multilevel society, a nested assemblage of multiple social levels between many individuals. Our aim was to build a model to explain the synchronization of activity in a multilevel society of feral horses. We used multi-agent based models based on four hypotheses: A) independent model: horses do not synchronize, B) anonymous model: horses synchronize with any individual in any unit, C) unit-level social model: horses synchronize only within units and D) herd-level social model: horses synchronize across and within units, but internal synchronization is stronger. We simulated each model 100 times and compared simulations to the empirical data obtained from drone observations. The hypothesis D best supported empirical data. This result suggests that synchronization occurred at an intra- and inter-unit level.