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 classical way to pluralize "–al" words:
un animal → des animaux [en: animal(s)]
un journal → des journaux [en: journal(s)]
with some exceptions: un carnaval → des carnavals [en: carnival(s)]
un festival → des festivals [en: festival(s)]
un idéal → des idéals (OR des idéaux) [en: ideal(s)]
un val → des vals (OR des vaux) [en: valley(s)]
There is no logic there (as many things in french), it's up to Mistral to choose how the plural can beEDIT: Format + better examples
I don't know what the precise rules or patterns actually might be. But one fact that jumped out at me is that -mal and -nal start with nasal consonants and three of the "exceptions" end in -val.
Back to this precise one, there's no precise rule or pattern underneath, no rhyme or reason, it's just exceptions based on usage and even those can have their own exceptions. Like "idéals/idéaux", I (french) personally never even heard that "idéals" was a thing. Yet it is, somehow : https://www.larousse.fr/dictionnaires/francais/idéal/41391
To my knowledge there aren't that many languages that are managed as officially as French is.
The Académie tried to codify what was used at the time (which varied a lot) to try and create a standard, but that's why there's so many exceptions to the rules everywhere : they went with "tradition" when creating the system instead of logical rules or purer phonetical approach (which some proposed).
There's a bunch of info on the wikipedia link about it, and how each wave or "réforme" tries to make it simpler (while still keeping the old version around as correct).
Each one is always hotly debated/rejected by parents too when they see their kids learning the newly simplified rules.
Recently, the spelling of onion in french went from "Oignon" (old spelling with a silent I) to "Ognon" (simplifying it out), and event that one made me have a "hmm" moment ;)