Horses (Equidae) are believed to formidably demonstrate the links between ecology and social organization. Their social cognitive abilities enable them to succeed in many different environments, including those provided for them by humans, or the ones domestic horses encounter when escaping from their human care takers. Living in groups takes different shapes in equids. Their aggregation and group cohesion can be explained by Hamilton`s selfish herd theory. However, when and which group to join appears to be a conscious individual decision depending on predatory pressure, intra group harassment and resource availability. The latest research concerning the social knowledge horses display in eavesdropping experiments affirm the need for an extension of pure genetic herd concepts in horses for a cognitive component. Horses obviously realize the social composition of their group and determine their own position in it. The horses` exceedingly flexible social behavior eagerly demands for explanations about the cognitive mechanisms which allow horses to determine their individual decisions.