There are a number of 13th-century examples
"Inter canem et lupum" is a real expression in medieval Latin.
I was unable to confirm that "Infra horam vespertinam, inter canem et lupum" occurs in the 7th-century Formulary of Marculf. The quote is almost identical to one attributed by Henry Spelman to an English legal text dated (if I'm reading Spelman correctly) sometime during the reign of Edward I of England ("infra horam vespertinarum, inter canem et lupum"). (Sebastian Koppehel's comment links to a source that cites other examples of the expression from roughly the same time and place.) As portions of this expression seem to be formulaic, it is not inconceivable that "inter canem et lupum" did occur centuries earlier in the Formulary, but as noted in the next section of this answer, I've had no luck finding it there yet.
Inter Canem et lupum ] Hoc est: Sub crepusculo, cùm lupus à cane non fit cognoscibilis. Placita coram Rege Eboraci in craft. Paschæ An. 31. Edouardi I: Rot. 16. Derb. in appello de morte viri sui, prosequuto per Marger. quæ fuit vxor Io. fil. Ric. de Benteley, vers. Ric, fil. Nic. Flerond, et Will. filium eius. Tali die infra horam vespertinarum, inter canem et lupum, venerunt et interfecerunt dictum Iohannem. Item aliàs. Brito Armoricanus Philippid. lib, 3.
Postea vix summos aurora rubescere montes
Fecerat, et valles nondum primordia lucis
Attigerant, interq; canem distare lupumque
Nullus adhuc poterat aliquo discernere visu.
Archaeologus in modum glossarii ad rem antiquam posteriorem, by Henry Spelman (1626).
On Google Books, I found an example of the phrase being used in a entry from the coroners' rolls of Bedfordshire in September 8, 1267:
Contigit apud Honidene in parochia de Etone die Jovis prox' ante Nativitatem Beate Marie anno 1o primo quod vj. latrones venerunt apud Honidene contra horam vespertinam inter canem et lupum et invenerunt unum garcionem Philippum filium Rogeri Golde qui venit de falda patris sui et ipsum verberaverunt et maletractaverunt et wlneraverunt et fecerunt ipsum vi ducere eos ad domum Radulphi filii Galfridi de Honidene, et vocaverunt dictum Radulphum dominum domus ad introitum habendum. Dictus Radulphus congnovit dictum Philippum garcionem et fecit hostium aperire.
It happened at Honeydon in the parish of Eaton on Thursday 3 next before the Nativity of the Blessed Mary in the fifty-first year that six thieves came to Honeydon about the hour of vespers at twilight, and they met a boy, Philip, Roger Gold's son, who was coming from his father's fold; they beat him, maltreated and wounded him, and forced him to lead them to the house of Ralph, son of Geoffrey of Honeydon, and they shouted to the said Ralph, lord of the house, to let them in. Ralph recognised the [voice of] the boy Philip and opened the door.
There are some other examples of the phrase in that book ("Contigit in villa de Ronhale anno 1o primo ad domum Simonis Ruffi die Sancti Petri ad Vincula quod Rogerus de Bemfeld et Andreas Beufrere de ..seye et alii felones et latrones duodecim vel plures venerunt ad domum Simonis Ruffi inter canem et lupum et intraverunt et ipsum Simonem in domo sua invenerunt..."). I just copied the text from Google's scan, so I haven't checked these for scannos.
Is it in the Formulary of Marculf? I'm not sure.
Other sources available on the web attribute "Infra horam vespertinam, inter canem et lupum" to the Formulary of Marculf; e.g. the scan on Wikisource of Dictionnaire des proverbes.
However, I haven't been able to find this text in the Formulary of Marculf, so I'm not sure there hasn't been a mixup somewhere with the attribution of the quoted text. Some references instead cite a phrase with the same wording except for starting with "Infra horam vespertinarum".
You can see a scan of the formulary on archive.org here. This is in Migne's Patrologia Latina, vol. 87, col. 691 to 967 (as mentioned by Rio). I didn't find any hits when searching the scanned versions of this linked here and here, but the scans are poor quality.
Rio's translation is based on Uddholm’s edition ("Uddholm, A., ed., Marculfi formularum libri duo (Uppsala, 1962)") and Rio also includes Zeumer in her bibliography ("Zeumer, K., ed., Formulae Merowingici et Karolini Aevi, MGH Leges V (Hanover, 1886)").
That Wikipedia article is about an unrelated Marculf: the death date of 558 given by Wikipedia clearly excludes the possibility of Saint Marcouf, abbot of Nantus in Coutances, Normandy, being the author of a 7th-century formulary. It seems the formulary was written by a different monk named Marculfus (note that the blog post refers to "Marculfe, abbot of Saint-Denis around the 7th century"). You can read more about the authorship in the introduction to Alice Rio's 2008 translation (Rio says "Marculf was apparently not a particularly uncommon name" -page 108).
Aside from Rio, you can see some information about the formulary and its author on the following web page "Selections from the Formularies of Angers and Marculf: A Translation with Notes", by Jane Jacoby (Persephone: The Harvard Undergraduate Classics Journal, Vol. 1, No. 1, Winter 2016). The dating of the document is a little uncertain, but generally seems held to be between 650-721 (my summary based on Rio, pages 110-113).
Du Cange's entry for canis seems to cite Spelman's quotations, referred to as "Willelmus Brito lib. 1. Philipp" and "Vetus Charta apud Spelman".
Vocabularium, seu Lexicon ecclesiasticum latino-hispanicum...,
by Rodrigo Fernández de Santaella (1789) says
Inter canem, & lupum. Este modo de hablar entre los Eclesiasticos Escriptores, es el crepusculo; aquella luz ambigua al amanecer, ó anochecer, que no se puede discernir si es lobo, ó perro. Entre dos luces solemos decir: Infra horam vespertinarum inter canem & lupum venerunt, & interfecerunt. Marcul. Vide Hiero. lexicon crepusculacens.
This appears to be the same quotation that we saw in Spelman, now attributed to "Marcul." Kind of suspicious, but I'm not sure exactly what's going on here.
The idea occurs in 2nd-century Jewish texts (not written in Latin)
The entry for canis in Walther von Wartburg's Französisches Etymologisches Wörterbuch says that it is proverbial in 2nd century Hebrew texts, and also in later Arabic ones, to use the inability to distinguish between a dog and a wolf as a criterion for the boundary between day and night:
Auch ALLo 18, pg. entre o cão e o lobo, sp. lubricán.
Die redensart ist morgenländischen ursprungs. Schon im 2. jh. nach Chr. ist in hebräischen texten zu lesen, „wenn man zwischen wolf und hund unterscheiden kann“, als bestimmung des zeitpunktes, an dem tag und nacht ineinander übergehen. So auch später bei den Arabern. Cuervo R 12,110; Schuchardt Z 28, 98 ; 29, 622.
(Vol. 2, page 197)
"My Journey Studying 2,711 Pages of Talmud" (by Shmuel Rosner, Jewish Journal, January 2, 2020), mentions this criterion, and someone who said it:
In the Talmud a dog appears for the first time on the second page of the first tractate. Each night, says rabbi Eliezer, consists of three watches, of which the second begins when the dog barks. The second appearance of the dog is six pages later, and it presents an interesting twist. While Rabbi Eliezer utilizes a dog to separate the second watch of the night from the first watch – rabbi Meir utilize it to recognize the coming of a new day. When is the exact time in which the day begins? “Rabbi Meir says when one can distinguish between wolf and a dog”.
This seems to be the relevant quote from Berakhot 9b:12:
רַבִּי מֵאִיר אוֹמֵר: מִשֶּׁיַּכִּיר בֵּין זְאֵב לְכֶלֶב.
It is talking about the morning, not the evening. I'm not sure whether the language is Hebrew or Aramaic.
Apparently, a translation of the Talmud from Hebrew and Aramaic into Latin was completed in Paris in 1245 ("Extractiones de Talmud").
Corpus Corporum search: I only see coincidental uses
I searched the Corpus Corporum, but the only example I found there (from around 1264, by Vincent of Beauvais) refers to a literal hybrid between a dog and a wolf:
Vincentius Bellovacensis, Speculum doctrinale (1264): LIBER DECIMUSQUINTUS., DE LICHAONE ET LINCISCO ET LINCE ET LOCUSTA XCII.
Linciscus ex cane et lupa nascitur, est enim uterque parens adeo luxuriosus, ut odio naturali sibi contradicente, quod est inter canem et lupum, in copulam tamen conveniant, propter luxurie delectamentum;sicque fit ut fetus contrahat sibi colorem et mores utriusque.
