"small-scale, petty" 1853, American English, colloquial, in reference to towns; see one + horse (n.). Probably from earlier use in reference to a carriage, sleigh, plow, etc., "drawn by a single horse" (1750); also "possessing only one horse" (of a farmer); hence "petty, on a small scale, of limited capacity or resources; inferior."
Shortly afterwards I took a stroll over the town. It was what is generally denominated a "one horse town," and I would think a pretty small pony at that. Two stores, one grocery, a stable, and four dwellings made up the sum of its buildings. ["Daguerreotyping in the Back Woods," in Yankee Notions, March, 1855]
"morning light, dawn," late 14c., from Latin Aurora, the Roman goddess of dawn, from PIE *ausus- "dawn," also the name of the Indo-European goddess of the dawn, from root *aus- (1) "to shine," especially of the dawn (source also of Greek ēōs "dawn").
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit usah "dawn;" Greek ēōs "dawn;" Latin Aurora "goddess of dawn," auster "south wind;" Lithuanian aušra "dawn;" Old English east "east."