In Frédéric Mistral's epic poem "Lou Pouèmo dóu Rose" (The Poem of the Rhône) from 1897, in Song XI, Verse XCVI is mentioned a creature. Here are the relevant lines in the original Provençal, in French, and in English (my own translation):
I'an vist pamens uno grosso Tartugo, I'an vist lou Lert, e d'autri bèsti orro... On y a vu pourtant une grosse tortue, on y a vu le Lert, d'autres bêtes horribles... However, there we saw a big turtle, there we saw the Lert, other horrible beasts...
In my copy there's a footnote associated with "Lert", which reads:
Le Lert, grand reptile fantastique
And from the context of the poem too, it seems it's some kind of dragon or dragon-like creature of local legend, but I can't find any mention of it outside of this poem.
Does anyone know a source with more details on this legendary creature?