http://www.tngenweb.org/civilwar/csacav/csa3forrest.html
3rd (FORREST'S OLD) TENNESSEE CAVALRY REGIMENT
This regiment had a very complex history, some 20 to 25 companies being at one time or another attached to it, with numerous changes in company letters.
CAPTAINS
• N. C. Gould, Co. "D". A Texas company; surrendered at Fort Donelson, later became Co. "K", 23rd Texas Cavalry Regiment.
As is well known, Colonel Forrest refused to surrender at Fort Donelson and led his force out through the backwaters of the Cumberland River, although two companies remained in the fort and surrendered. These were Captain Gould's Company "D" and Captain Davis' Company "I. Neither of these companies again served with the regiment.

Paraphrased from Compiled Service Records of 23rd Texas Cavalry Regiment

6/24/62 Col Gould cdg Regt to Sec Randolph, Richmond-After the capture of my company at Ft Donelson, Col Forrest ordered me on recruiting service & afterwards Gen A S Johnston authorized me to recruit a mounted Bn for the war. I have so far carried out these orders that I have recruited a Regt which is now camped in Red River County.
I should have reported for duty to some cdr could I have obtained arms west of the Ms.
I wish this regt rec’d as Partizans
Would like the opportunity to explain more fully in person

10/25/62 The 23rd Texas Cavalry regiment was organized for 3 years

Co K-9/13/61-from Forrest’s old cavalry regt

11/26/62 Col Gould to Gen Magruder-I have encamped at this place a regt of Cav and that we are ready to march well equipped save arms which I suppose are in or soon to be in Shreveport.
Our horses are much better than most of the other Cav regts raised in this state. Our men are all inured to the hardships of camp & have had the measles
Our QM has paid money for what we have so no time will be lost in that regard. One Co was with then Col McCulloch last year. One was with then Col Forrest in Ky
I await orders from your HQs for me to march immediately if you can consistently give them. I do not know whether I am in your Dept or not-you of course do. If I am not will you please communicate a wish to Gen Holmes for the ____ [rest-?]