Ms. Marriott,

The 1st Texas Lancers were also known as the Texas 21st Cavalry Regiment. The unit was organized in early 1862 . Their first commanding officer was Col George Carter. Other officers were Major Chenoweth, LtCol D.C Giddings, LtCol Robert Neyland and Major William Walton. According to Sifakis , the 24th and 25th Texas cavalry Regiments were formed out of this (21st) regiment at Hempstead , TX ( northwest of Houston , Tx about 60 miles ..my notes, not Sifakis) on April 24, 1862.
The 21st fought in the Trans Mississippi theater. They fought in the Little Rock Campaign ( August -Sept, 1863), pine Bluff( Oct 25, 1863) , and in the Red River Campaign ( March - May , 1864) . For further reading , try Ms Anne Bailey's " Between the Enemy and Texas: Parson's Texas Cavalry in the Civil War" .
Good Hunting , Craig