Hazel, at more than 20 years old, was one of the oldest mares on the beach, according to the Corolla Wild Horse Fund.
The non-profit noted that the horse had no signs of trauma, major injury or illness that would indicate she didn’t die of natural causes.
“We had noticed that she’d been slowing down some lately. The heat has been really hard on the older horses,” the group wrote on Facebook.
On Friday, the National Weather Service issued an advisory for the Outer Banks that temperatures could reach up to 110 degrees Fahrenheit (43C). Saturday reached a high temperature of 99F, and Sunday 100F.
“She seemed to enjoy her role as honorary grandmother to the foals, and could be seen babysitting while their moms grazed and rested,” the nonprofit wrote on social media.
“We will miss seeing her on the beach but take comfort in knowing she lived a great life and left a huge mark on the herd.”
The North Carolina area is home to a herd of wild Colonial Spanish Mustangs, descended from the horses brought to the Outer Banks by early colonisers. They roam freely on the beaches and eat sea oats and native grasses. Other wild horse populations on the East Coast can be found on Assateague Island, a long barrier island in Virginia and Maryland.
Conditions can be tough for the horses, as the charity notes. Between 1985 and 1996, 20 horses were killed by vehicles, and they must also contend with hurricanes and food scarcity.