This is the only Zach Higdon I find, Confederate/Union.

Zach T. Higdon

Residence was not listed;
Enlisted as a Private
"G" Co. MO 18th Infantry, USA

Sources used by Historical Data Systems, Inc.:
- Index to Compiled Military Service Records

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Zach/Zachariah T. Higdon, Private, Company G, 18th Regiment Missouri Infantry, USA, enlisted December 7, 1864, at St. Joseph, Mo., 7th District, as substitute for Jacques Heckinlively, mustered out July 18, 1865 at Louisville, Ky., description: 18 year old Farmer, born in Kentucky, blue eyes, sandy hair, fair complexion, 5' 5-1/2", signed enlistment document with an 'X', "his mark"

M405: Compiled Service Records of Volunteer Union Soldiers Who Served in Organizations From the State of Missouri.