The quintessential example is "cheval" (horse) which becomes "chevaux" (horses), which is the rule they're following (or being cute about). Un mistral, des mistraux. Un ministral, des ministraux.
(Ironically the plural of the Mistral wind in the Larousse dictionnary would technically be Mistrals[1][2], however weird that sounds to my french ears and to the people who wrote that article perhaps!)
[1] https://www.larousse.fr/dictionnaires/francais/mistral_mistr... [2] https://fr.wiktionary.org/wiki/mistral
The only plural form people will probably know is from the song Mistral Gagnant where the lyrics include les mistrals gagnants but that refers to sweets!
Not sure why anyone would think "les mistraux"... ;)
https://en.m.wikipedia.org/wiki/Mistral-class_landing_helico...