sulky

noun, a light two-wheeled horse-drawn vehicle for one, esp. used in harness-racing (COD); also attributive as below

“Tom’d pick him up and give him a ride on the sulky. And of course by then the horse was tired and wasn’t going to be running fast or going crazy.”

“These were sulky horses with the carts.”
“Sulkies, okay, sulkies.”

“I always had racehorses… It was harness racing, so we actually had the sulky and the driver.”
“What’s it called, sulky?”
“Sulky racing, harness racing.”