a fabulous monster, with the head and fore quarters of a horse joined to the tail of a dolphin or other fish (hippocampus brevirostris), -- seen in pompeian paintings, attached to the chariot of neptune.-a genus of lophobranch fishes of several species in which the head and neck have some resemblance to those of a horse; -- called also sea horse.-a name applied to either of two ridges of white matter in each lateral ventricle of the brain. the larger is called hippocampus major or simply hippocampus. the smaller, hippocampus minor, is called also ergot and calcar.