There's a pretty great movie about this story; https://en.m.wikipedia.org/wiki/Brotherhood_of_the_Wolf

There is also power metal https://www.youtube.com/watch?v=po-u-V6GiEk

Is it possible it was just an escaped lion? Has a lion ever lived in the wild in France?

Quoting myself from a few months ago:

French here, never heard of any connection with werewolves. Only that it was either an immense wolf or possibly a hyena brought who-knows-how from Africa.

Note from a dive in the rabbit hole of the French Wikipedia page: it seems it was truly a wolf which was - according to its autopsy - 1m long "from the base of its tail to the top of its head" and weighed more than 50kg.

50kg is big, but not a monster wolf. Go to any zoo or sanctuary and you will see grey wolves that size. Some places will let you pet them. Imho the more likely candidate is a wolf-dog hybrid as they can be considerably larger than either parent.

Maybe some wigbrained aristocrat got one for his menagerie. Alternatively:

https://en.wikipedia.org/wiki/Phantom_cat

The article mentions that under hypotheses. Not sure if sibling comment is being tongue in cheek but the article mentions menageries specifically

many lions lived in France when France occupied Africa ;)

This is a common trope, but I do always love it when old drawings were clearly made by someone who had not seen what they were drawing. All the depicted people look very real and proportional while the “beast” is comically bad

This is a major plot device in Teen Wolf during the later seasons.