[Sword & Shield] How to catch???

At the end of the overworld just before the stairs, there is a viewing point where there are 2 Pokemon flying near the cliffs, it has a path (but you can't go down it) going down the mountain, and they are flying just out of reach to react to a whistle. They look like Frosmoth but different (larger and darker). How do you get an encounter with them????

Sorry, path to the peak.

Those are definitely Frosmoth. If they're too far to whistle and never come close enough to whistle, then they might just be part of the environment and not encounterable ones.