The 14th Arkansas Infantry Regiments were Confederate.
There were actually two units that bore the 14th Designation. The 1st 14th, McCarver's was merged with the 17th (LeMoyne's to form McCarver's 21st Arkansas Infantry. the 2nd 14th Infantry is known as Power's 14th Arkansas. Power's Regiment fought at Pea Ridge, the 14th/21st fought at Vicksburg.

Compang G Power's 14th Arkansas:
Company G of this regiment was recruited in Carrol County, Arkansas. A West Matlock is listed on the roster, having enlisted at Yellville. an Issac Matlock is listed in Company B, which was recruited in Marion County he enlisted at Yellville, later he deserted and joined the 4th Arkansas Infantry US Army (Union).

Company G McCarver's 14th: did not have any Matlock's listed.

However, Company I had a J.W. Matlock enlisted Pocahontas Oct 1861 age 33. A J.B. Matlock age 18 enlisted Pocahontas, Oct 1861.

Several John Matlock's are listed in various Arkansas units. To pinpoint your ancestor, addtional information would be helpful.

Do you know what your John Matlock's middle name was and do you know what county he was living in in 1861?