He can show up at the Campfire Tales event as the final boss.

It can also be the final boss of daily ops with alien enemies or they spawn in third wave of the alien invasion hourly events.

any clue if invasion will come back like fascnacht?

It will. But will probably be a few months. Both alien invasion and mothman equinox are supposed to happen twice a year but this year has been extremely goofy so who's to say if we will get another one this year. If we do it will probably be between Halloween and Xmas but I'm not gonna bet on that.

I agree with the campfire tales. Also, they tend to appear often if you adventure around the mire and savage divide. They are one of the spawn pools for the Harper's Ferry train station locale, at the tunnel entrance they can spawn on the top of the mountain.  Just south of Freddy Fear's House of Scares, in the wilderness area you can also find one sometimes. Pretty much any time you encounter a Mothman in the wild, aside from Cultist camps, there's a chance for a flatwoods monster to spawn instead.

I ran into one recently in the north end of skyline valley.

Theres one that sometimes spawns near whitespring

Raider Punk camp ally will sometimes give you a "Kill the flatwoods monster" quest. This is how I finished the challenge.

I've had him for awhile but I just get kill sheep squatch and weapon case

Random spawn. I haven't seen him in the wild since reclamation day