R.G.M. Nisbet and Margaret Hubbard, A Commentary on Horace: Odes, Book I
(1970; rpt. Oxford: Clarendon Press, 2001), pp. 241-242 (on 1.19.12):
Parthum: the collective singular is an old construction in Latin; Varro mentions that 'Romanus sedendo vincit' was a 'vetus proverbium' (rust. 1.2.2). It seems to have been both archaic and colloquial, and is consequently affected by the historians (except Sallust) and avoided by Cicero (except in his letters); cf. K.-S. 1.67f, K.-G. 1.14, H.-Sz. 13f., Norden on Virg. Aen. 6.851, and especially Löfstedt, Syntactica I2.12ff. It is particularly common in military contexts, where it is convenient to think of a mass rather than a collection of individuals. A refusal to individualize is especially useful when thinking of enemies, and hostis and the names of hostile peoples are consequently very often employed in the singular. Cf. also Rose Macaulay, Staying with Relations, p. 19 'Squealings and tramplings came from the forest on the left. Isie said, "Plenty of pig in there." And Catherine surmised, from her use of the singular number, that she would fain pursue these animals and take their lives.'