There are two populations of "ponies" on Assateague. A VA herd and a MD herd. The land-manager for each is different (USFWS vs NPS). For the VA (USFWS) herd,

There are two populations of "ponies" on Assateague. A VA herd and a MD herd. The land-manager for each is different (USFWS vs NPS). For the VA (USFWS) herd, the herd size is capped at 150 adults and the Chincoteague VFC is the herd manager/owner. The annual pony crossing is part of herd health and population management. Pregnant and newly birthed mares/foals do not make the crossing - they are corralled separately and transported via trailer. Part of the crossing effort includes health checks as well as the auction. The MD (NPS) herd is feral with contraceptives used to manage population and avoid over-grazing. Other than contraception, the herd is managed like other wild life (ie, mostly left alone, except for exceptional circumstances). This herd is not part of the crossing. There are several wikipedia entries for the ponies, the crossing, etc, if you're interested.