You can find the saying lupus in fabula in Terence and in Cicero, and also in Plautus, in the form lupus in sermone, "the wolf in the talk."
Aeli Donati commentum Terenti, vol. II, recensuit Paulus Wessner, editio anni millesimi nongentesimi quinti (1905),* wrote:Lupus in Fabula silentii indictio est in hoc proverbio [Terenti, Adelphoe] atque eiusmodi silentii, ut in ipso verbo vel ipsa syllaba conticescat, quia lupum vidisse homines dicimus, qui repente obmutuerunt; quod fere his evenit, quos prior viderit lupus, ut cum cogitatione in qua fuerint etiam verbis et voce careant. nam sic Theocritus (Id. XIV 22) "οὐ φθεγξῇ; λύκον εἶδες" et Vergilius (Ecl. IX 53-54) "vox quoque Moerim iam fugit ipsa, lupi Moerim v[idere].p[riores].". alii putant ex nutricum fabulis natum pueros ludificantium terrore lupi paulatim capua venientis usque ad limen cubiculi, nam falsum est quod dicitur intervenisse lupum Naevianae fabulae Alimonio Remi et Romuli, dum in theatro ageretur.
"The wolf in the story" is an indiction of silence in this saying [in Terence's Adelphoe] and moreover of such a kind of silence that he [the speaker] should right in a word or in a syllable cease to talk, because we say that men have seen a wolf who suddenly become silent; insofar as it happened to them that a wolf just earlier had seen them, that with the recognition of their situation even their words and voices failed them. For thus [says] Theocritus (Idylls. XIV 22) "‘Do you say nothing?’ says one, and joking, 'Have you met a wolf?'" and Vergilius (Ecloga nova, versus quinquagesimus tertius et quartus) "and even his voice has deserted Moeris, the wolves having seen Moeris first". Others reckon [it's] from the stories of nursing nannies to frighten boys for their carryings on by the coming of a wolf bit by bit from Capua right up to the bedroom threshold; on the other hand, it's not the case what's said that the wolf had come from the "Upbringing of Remus and Romulus" of Naevius's play, when it used to be performed on the stage.