COROLLA, N.C. — Come summer, the beaches of this barrier island will be choked with cars and sunbathers, but in the off-season the land is left to wild horses. Smallish, tending toward chestnut and black, they wander past deserted vacation rentals in harems of five or six.

Thousands of them once roamed the length of the Outer Banks of North Carolina, the likely descendants from mounts that belonged to Spanish explorers five centuries ago. Now their numbers have dwindled to a few hundred, the best known living on federal parkland at Shackleford Banks.

But the largest herd, which has recently grown to almost 140 strong, occupies more than 7,500 acres of narrow land that stretches from the end of Highway 12 in Corolla (pronounced cor-AH-la) to the Virginia border, 11 miles north. Lacking natural predators, and trapped by fences that jut into the choppy Atlantic, the herd is becoming so inbred that its advocates fear a genetic collapse in mere generations.

These supporters are leading a campaign to save the Corolla herd, and they have powerful allies in Congress. In February, the House passed a bill that would sustain the herd at about 120 and allow the importing of new mares from Shackleford for an introduction of fresh genes.