Documents show they owned two slaves.
Nancy Jane McNeil arrived to Texas in 1821 with her husband Robert Hemphill Millican, and their eight sons and two daughters. On 1824, Robert Millican, originally from Spartanburg County, South Carolina, received a land grant from Stephen F. Austin in the big bend of the Brazos River in what is now Brazos County. The census of March 1826 classified him as a farmer and stock raiser, and his household included, apart from his family, two slaves. Mrs. Millican became a widow in 1836 and remarried to William S. Ellis on June 1838.
Elliot McNeil Millican (1808-1860) received a land title adjoining his father’s grant on March 1831. Between 1839 and 1844 he served as constable of Washington, sheriff of Navasota County, and sheriff of Brazos County, until he was elected representative for Brazos County to the Ninth Congress of the republic (1844-1845). During the Fifth and Sixth legislatures, he served as senator from Brazos County. He resigned from the Senate during the sixth session because of a widespread epidemic, as he was one of the few resident physicians in Brazos County. He devoted himself to his medical practice until his death during a cholera epidemic on October 13, 1860.
Lon F. Curbello, Jr., "MILLICAN, ROBERT HEMPHILL," Handbook of Texas Online, accessed September 04, 2015. Uploaded on June 15, 2010. Published by the Texas State Historical Association.
"Millican, Robert Hemphill (1750–1836).Robert Hemphill Millican, one of Stephen F. Austin's Old Three Hundred colonists, the son of William and Mary Millican, was born in Spartanburg County, South Carolina, in 1750. He married Nancy Jane McNeil in the 1770s and arrived in Texas in December 1821. He brought two married sons, William T. and James D., with their families, a widowed daughter with her family, six other sons, and one daughter. Millican received title to 2½ sitios of land in the big bend of the Brazos River in what is now Brazos County on July 16, 1824. Horatio Chriesman presented his bill for surveying the land in October 1824. Sometime before November 1824 Millican's home was the site of an election of militia officers. The census of March 1826 classified him as a farmer and stock raiser aged over fifty; his household included his wife, Nancy Jane (over fifty), four sons, two daughters, three servants, and two slaves. The census carried a note that he was building a grain mill on the Brazos River. Millican died during the Runaway Scrape in April 1836, of pneumonia and measles, on the bank of the Trinity River just above the town of Liberty. He was buried there on the riverbank. He was survived by his wife, eight sons, and two daughters. Mrs. Millican married William S. Ellis on June 11, 1838."https://www.tshaonline.org/handbook/entries/millican-robert-hemphill
His sons may have owned slaves, so you may want to look into those.