The poem published as "A Visit from St. Nicholas" is usually credited to Clement Clarke Moore, but it's very possible that we will never know for sure who wrote it.
When it was first appeared in the newspaper on December 23, 1823, there was no name attached to it. It wasn’t until 13 years later that Clement Clarke Moore, a professor and poet, stepped forward to claim authorship and said that his housekeeper had, without his knowledge, sent the piece he wrote for his kids to the newspaper. In 1844, the poem was officially included in an anthology of Moore’s work. The problem? The family of Henry Livingston, Jr., claimed their father had been reciting “A Visit From St. Nicholas” to them for 15 years before it was published. Here’s the view from both sides.
The evidence in an article at mental_floss seems to point to Livingston, with even more in the comments. Link