Glenda -

This from Ralph Wooster's "Lone Star Regiments in Gray" from Eakin Press, Austin, TX:

The 25th Cavalry (dismounted) served in Robert R. Garland's brigade at Arkansas Post in January 1863. When Arkansas Post fell, many were sent to prisoner of war camps in the north. The unit was reconstituted after many of the soldiers were paroled in April 1863. The men were then placed in a brigade commanded by Brig. Gen. Thomas J. Churchill and fought east of the Mississippi.

You should get a copy of your ancestor's service record to do further research.

Hope this helps,
Thomas Reid