It's my first real explanation in this sub, I hope I did something correct. If not please advise me.
I can provide some information, but unfortunately most of my sources will be in French, for obvious reason.
First of all, I'll just put a bit of a background for others. La bête du Gévaudan (la bèstia de Gavaudan in Occitan) was an animal which attacked people between 30^th June 1764 to 19^th June 1767. In the region called Gévaudan an old French area which is now somewhat Lozère. South of France.
Most descriptions are from that era. The beast was seen a lot of time: notably it attacked young boys in 1765 who defended themselves against it, this is the famous Combat de Portefaix (Portefaix's fight).
I'll be quoting the proceedings after it was killed Flat mouth and fine snout, short ears, grey-red hair crossed by black bands, white mark on the chest. 109 livres (~53,4 kg), size (head to tail) 3 feet (97,44 cm), "opening of the mouth" 7 pouces (18,9 cm) p. 608-609 of the Chronology by Alain Bonet.
About what it was now.
Connoisseur and hunter agree about a fact: they have never seen any wolf like that. Also the beast was called Bèstia (beast) and not wolf.
It cannot be a common wolf (Canis lupus lupus) since people knew very well about them.
The beast killed most probably is the Beast of Gévaudan, most descriptions of the beast are accurate to the one of the dead beast: size, colour, wounds and... they found humans rest in its stomach.
Common explanation at that time, and kept by historians, is the fact it's a mix between a wolf and a dog. It's common at that time. In 2016 a reconstitution was made following the description in the proceedings here.
In a letter wrote maybe by a practitioner [...] its physiognomy being softer than a wolf, I think it's a mulet [hybrid] between a wolf and a dog, or a dog and a she-wolf. (p. 612).
Another theory: it was an exotic animal. Since the beginning, when the captain Duhamel (captain of the Dragons) came, he wrote in his letters the possibility of being an African beast like a leopard or a lion. The notable beast was the hyena which could have escaped from some fair. A theory about the lion on NationalGeographic. I'm not a reliable source by far, and I think it's not really allowed(?), but I don't find it really convincing myself, the comment of Rob explain why, Karl answered this but didn't convince me though.
Sources:
Amazing chronology and lot of information in this .pdf , compiling a lot of different sources in French (656p), but Alain Bonet.
La bête du Gévaudan, G. Lenôtre ISBN 979-10-94569-27-6
Comptes des frais de chasse de la Bête du Gévaudan (1765), et d'un loup qui désolait les diocèses de Grasse et de Fréjus. (1788-1789). On Gallica.
Website about the Beast I wanted an English "source" so I put that. You have translated extract of documents. It doesn't explain really what was the beast, but you'll have some information which can be helpful.
If you have any questions don’t hesitate.