Their Company F men most likely formed up at Rockett and were brought together at Hempstead with the other companies from various counties to form the regiment.

12th Cavalry Regiment was organized with about 940 men in August, 1861, by Colonel W.H. Parsons. Most of the men were from Hempstead, Fairfield, Georgetown, and Waxahachie, and Ellis and Hill counties.

The regiment organized for state service on September 11, 1861, at Rockett Springs near Waxahachie.
https://tshaonline.org/handbook/online/articles/qkt12

It was first sworn in as state troops for one year.

The companies that made up the Twelfth Texas were:

Company A, the Hill County Volunteers, recruited in Hill County;

Company B, the Freestone Boys, from Freestone County;

Company C, the Johnson County Slashers, from Johnson County;

Company D, the Bastrop Cavalry Company, from Bastrop County;

Company E, the Ellis Grays, from Ellis County;

Company F, the Ellis Rangers, made up of the companies of the Texas Mounted Guards from Ellis County and the Texas Mounted Rangers from Parker County;

Company G, the Kaufman Guards, from Kaufman County;

Company H, the Ellis Blues, from Ellis County;

Company I, the Williamson Bowies, from Williamson County;

Company K, the Eutaw Blues, from Limestone County.