When you enter so many forests, you see many Snivy's and Servine's living in the woods but not their final forms. That's where you encountered one when one is approaching you

"Hello there, dear....what brings your peaceful soul to my forests?"

This "Lord of the Forest" is Serperior, a primarily pale green, serpentine Pokémon, who ruled these forests with a gentle care

"Is it cuz of something you want from me?"

Serperior asked as she coils around you