We numismatists have caused ourselves a world of unnecessary confusion by the common language of our catalogs that describe various deities as being in a biga or in a quadriga. In Latin in bigis just isn’t used. Perhaps because the visual conjured up by such a phrase might be something like the scene with Luke and Han on Hoth:
The phrase ‘in curru’ is regularly used. And we might note especially the line of Lucretius On the Nature of Things (2.601):
sedibus in curru biiugos agitare leones
There are four instances of in quadrigis in Latin, but notably three describe statues.
Gaius Plinius Secundus, Naturalis Historia 34.78.4
C. Iul. Caes. Augustus Octavianus, Res Gestae 4.51
Maurus Servius Honoratus, In Vergilii Bucolicon Librum 6.22.3
And the fourth is in Cicero’s Brutus when he means ‘in the chariot races’ not ‘in the chariot’ (173.5). [I leave aside the odd Latin of Hyginus, Fabulae 250].
bigae and quadrigae, as Latin grammarians are forever going on about, are plural nouns not singular, because they refer to the animals, not the vehicle. Perhaps the most clear is the statement by Fronto from Aulus Gellius’ Attic Nights:
The Loeb translation is misleading. So here’s a slightly modified version:
Quadrigae, etsi multiiugae non sunt, always keeps the plural number, since four horses yoked together are called quadrigae, as if it were quadriiugae, and certainly that which denotes several horses should not be compressed into the oneness of the singular number.
The problem is how to translate etsi multiiugae non sunt: ‘although they are not many’ is accurate, but misses the contrast in the Latin between quadrigae and multiiugus, the latter adjective which can be singular, where as the former cannot. Or we might even read a joke here, ‘although there are not a many teams yoked together’. But how funny was Fronto, really?
Anyway all of this is just in support of Luigi Pedroni’s point in AIIN 2010. p. 349:
“The term bigae, in fact, was originally used only in the plural, and this confirms that it simply indicate two horses paired and not specifically a chariot drawn by two horses, a concept that was extension of the original meaning. Catullus 29 makes this clear: “Rhesi niueae citaeque bigae”, where the nivae metonymy refers to the horses, their white color was proverbial, and not to the chariot of Rhesus.
It can be argued, therefore, that at the beginning of the second century. B.C. a bigatus was a coin with iconography depicting two horses: it is sustainable, moreover, that the term could also refer to mounted animals rather than yoked. Therefore, as suggested by Seltman previously (but with a different chronology), followed more recently by Harl, it may have been used to describe the Dioscuri who were often traditionally represented with their horses as a pair.”