The natural family group consists of a stallion (the herd sire), his mares and their joint offspring aged up to 2 years. The number of animals in these family groups rarely exceeds 20. Families consisting of the stallion and older mares naturally remain together for years. Depending on breed, colts and young stallions wander off of their own accord at an age of 1.5 to 3 years, or are chased away by the herd sire. Mares that are no longer capable of reproducing remain within the family group.

Colts and young stallions from various families form small bachelor herds of fewer than 10 animals, which may also remain together for years. These provide replacements for sick and ageing herd sires, their mares generally being taken over as a unit. Fillies in first heat sometimes join the young stallions, which will then fight over them intensely. Where stallions only run with the herd during the breeding season, the mares form matriarchal groups consisting of the senior dams and their daughters with foals. It is not uncommon for old and sick horses to form a “misfits group” for security.