dog soldier
/ˈdɒɡ ˌsəʊldʒə/originally and chiefly US
noun
1Chiefly with capital initials. A member of any of various (elite) military societies among certain North American Indian peoples of the Great Plains. Also occasionally in extended use: a Plains Indian warrior, especially one regarded as hostile or uncompromising. Now chiefly historical.
2 Military informal. A soldier of low rank; an infantry soldier, a ‘grunt’.
3A dog used by the military, especially in active combat.
Origin
Mid 19th century; earliest use found in Rufus B. Sage (1817–1893). From dog + soldier.