The Thoroughbred/Chincoteague cross has been popular for producing sport ponies. Most are considered partbreds but there's been some instances where they have become part of the Chincoteague bloodline. Thoroughbred blood can be isolated in DNA testing. The Chincoteague Ponies tested so far have had below 5% Thoroughbred blood.

Beaufort Farms of Harrisburg, Pennsylvania bred at least one Thoroughbred cross. The 1956 born colt Leap Year was sired by a Chincoteague stallion and out of a 7/8 Thoroughbred mare. He was described as a crossbred pony in Ponies for Young People by Ehrman Mitchell. It's unknown if Leap Year sired any foals and if they were considered Chincoteagues or partbreds.

Leap Year, from Ponies for Young People by Ehrman Mitchell.

Stormy's oldest daughter Windy, born in 1969, was sired by a Thoroughbred. Her Thoroughbred blood was confirmed by genetic testing of a descendant with multiple crosses to her. Her descendant had over 20% Thoroughbred blood, unusual for most Chincoteague Ponies. She was the result of an accidental breeding of Stormy with a racehorse trained by Ralph Beebe. His name is unknown at this time. Windy was "officially" listed in the Pictorial Life Story of Misty as being by a Chincoteague stallion named Lightning. A couple of the Thoroughbred stallions Ralph raced in the mid 1960s were were Dodger Artful and Take a Bow. Given how common outcrossing was at that time perhaps there was deliberate Thoroughbred crossing with some of Ralph's Thoroughbreds on Beebe Ranch's Chincoteague mares.

Unknown Thoroughbred that may be Windy's sire

Windy and her first two foals, from a Chincoteague Miniature Pony Farm postcard.

The Daily Notes, October 14, 1971.

A source quoted in The Wild Horse Dilemma by Bonnie Gruenberg stated a Thoroughbred stallion named Red lead a band of mares on Assateague in the 1960's. An October 22, 1967 Baltimore Sun article featured a tall stallion named Big Red in Maryland that appears to be that stallion. He had a herd of mares with the fire company's F brand. The F brand mares were retrieved by the CVFC in July 1966 and Red was left behind. Big Red had a PB brand, but perhaps that was actually an RB for Ralph Beebe and he was a former racehorse.

Big Red with his Maryland mares, from The Baltimore Sun

Misty's Twilight was the subject of a Marguerite Henry book and was sired by the Thoroughbred Big Bluffer. However she is generally considered to be a partbred. Misty's Twilight was a registered Pinto.