The review examines the historical aspects of the hippocampus nomenclature. Its anatomical study began by the ancient Greeks, who called the unusual–looking structure a “ram’s horn” – cornu ammonis. The term “hippocampus” (ancient Greek: ἱππόκαμπος, from ἵππος, “horse” and κάμπος, “sea monster”, or “sea-horse”) was first introduced in the sixteenth century by anatomist J.C. Arantius. The term “hippocampal formation” is currently applied to a group of cytoarchitectonically different adjacent areas, including, along with the hippocampus itself, the dentate gyrus, subiculum, presubiculum, parasubiculum and entorhinal cortex. The reason for including these six regions in the “hippocampal formation” group is that they are connected to each other by unique and largely unidirectional pathways. The review is devoted to the spatial, morphological and cyto- and myeloarchitectonic organization of all departments of the rat hippocampus formation and the distinctive neuroanatomic characteristics of its departments. Comparative features of the structure of the hippocampus formation of a rat, monkey and human are described. Although the volume of the hippocampus is about 10 times larger in monkeys and 100 times larger in humans compared to rats, the basic architecture of the hippocampus formation is common, although there are some species differences. The relatively simple organization of the main cellular layers in combination with the highly organized laminar distribution of hippocampal neuron processes contributes to its use as a model system in modern neuroscience.