Even when taking the flames on its back into consideration, the starter Fire-type Pokémon of Johto – Cyndaquil – looks more like a Grass-type than a Fire-type.

Source: https://www.cbr.com/fire-pokemon-that-should-be-diiferent-types/

Evolving from Sunkern when exposed to a Sun Stone, Sunflora may not be the most powerful Grass-type, but it's certainly one of the most iconic.

Source: https://gamerant.com/best-monotype-pokemon/

Its possible moveset includes the aforementioned Grass-type Trailblaze but also a few other Ice-type moves, such as Ice Beam and Blizzard.

Source: https://screenrant.com/greninja-tera-raid-counters-pokemon-scarlet-violet/

Only Fire-, Water-, and Grass-type Pokémon can enter, and they must have never evolved despite being able to.

Source: https://centralrecorder.com/starter-pokemon-takes-over-community-day-this-week-in-pokemon-go/