I tend to think of the Forestman as being the Errol Flynn "Adventures of Robin Hood" characters they resemble. However, the classic Forestmen have a kind of a uniform look that resembles "troopers" or "faceless legions" rather than characters. They're pretty generic.
Anyway, for me they can be adventurous swashbucklers hiding as fugitives from the law in the forests, or they can simply be civilians who work in the forests, like woodcutters and hunters. The only unifying theme I really have is that individual career as a "Robin Hood" style outlaw, a woodcutter or a huntsman aside ... all these guys are men of the forest, hence being grouped as "Forestmen".
They fill whatever role I need, like somebody else said, based on story and character, and whether I need outlaws, persecuted peasants who've fled into the forests for safety, or just normal hard-working people who live in the woods.
Over all though, I can say that they're good people. Desperate and uncouth sometimes, but good people. And yeah, I have an actual "Robin Hood" character (nameless at the moment). A dejected knight who's land was stripped by crooked marshalls of the crown.
Dark Forest naturally lends itself to "darker" motifs. These men either LITERALLY live in darker forests (evergreen forests higher in the mountains) or they live among the regular Forestmen, they just happen to be thieves and bandits.
Sometimes Dark Forest for me operates like a low-key criminal enterprise, a gang of thieves and miscreants who could give a bad name to the standard Forestmen. Sometimes they're just ... MORE Forestmen. But they're definately the lowlier, less cheerful bad eggs of the Forest dwellers.
(As for the Wolfpack ... thieves, cheats, killers-for-hire ... the Wolfpack to me is the organized crime of Castle. They're into extortion rackets, smuggling, raiding royal convoys and selling the swag, they'll do anything for their greed - even sneaking into heavily fortified keeps and castles to rob treasuries, and they're cunning and sneaky like the wolves they use as mascots.)