A frowning horse plush has become a viral bestseller ahead of Lunar New Year celebrations for the year of the horse in China.

The sad-looking soft toy was originally made in error after a worker sewed a smile on the horse upside down, the owner of Happy Sister - a shop in the eastern city of Yiwu - told Reuters news agency.

Zhang Huoqing had expected to have to issue a refund to the customer who bought the toy, but after an image of it started circulating online the horse began selling out.

Zhang said the animal's gloomy expression was resonating with young workers in China.

"A lot of customers like it, and they said it makes sense: that it suits the spirit of today's corporate slaves," Zhang said.