Explained by Deanna Dahlsad:
Some claim it is to validate and honor Jocko Graves, the son of a free black soldier named Thomas Graves, who fought with George Washington. The story goes that Washington assigned the youth to safely remain on the Pennsylvania shore with the horses while they crossed the Delaware. Jocko was also to keep a lantern burning so George and the soldiers would know where to return after battle. When Washington and his army returned they discovered Jocko had frozen to death — still holding the horses and the lit lantern.
The story continues that Washington was so moved by Jocko’s devotion that he commissioned a statue in Jocko’s honor. Titled “Faithful Groomsman” the statue stood at Mount Vernon in honor of the young patriot. ...
But Professor Kenneth Goings ... says this legend isn’t true. And in an October interview with ‘The Daily Journal’ Goings says the lawn jockeys are “very, very much racist symbols” and says that he’s amazed people can believe anything else. He continued to say black lawn jockeys are part of the Old South mythology: “They are meant to evoke that Old South, grand plantation, “Gone With the Wind” mythology, and I’m not sure they can evoke anything else.”