Frosmoth is available in Pokemon Sword and Shield's first DLC pack the Isle of Armor.
To find out how to evolve Snom into forsmoth, read below!
Night Friendship
According to Frosmoth's Bulbapedia page, you must use friendship to evolve Snom.
The friendship level must be high to begin with and Snom will only turn into Frosmoth if they level up at night.
