so going by what I know of 47's build of bogeymen it's more bogeymen can shapeshift into an animal which drops the count of bogeymen down which allows the cackling(the event that summons them) to fill in with more bogeymen until that bogeyman shapeshifts.
if you say have no fear they don't go defensive and turn into an animal unless you provoke them which then you only get one shapeshifting bogeyman who turn into an animal that can feel fear surrounded by a bunch of creatures who hunt that.
if you play a spider-person in those areas you get some fun encounters and performance buddies out of that... and or a necromancer you can get a zombie bogeyman mount if you get them out of the cackling.
the cackling also tries to summon up to a set number of bogeymen if you wander off from the other set which cause the ones too far away from you to violently explode into smoke dying.
so the ones that violently blow up aren't counted to the death toll so I take it if you kill a shapeshifted bogey that also doesn't count to the death toll but there's a silver lining in this.
if you're crazy devoted into bogeyman research and want to weaponize/use/see the potential of a shapeshifting 2 foot tall fluffy being be chill one could get them out of the cackling by ending the cackling event while they are shapeshifted which then they just remain after the event purge the visible bogeymen... leaving you the left overs who linger about the world to interact with said world.
they are sapient so they could end up joining a bandit group or a church... or a site as a guard or something lair focus.
oh yeah the zombie bogeymount is more making a zombie bogeyman and waiting for them to turn into a mountable animal and ride them, which when the transformation wears off you get the means to maintain the ride so one could just go about riding a bogeyman that could have flight and if a battle breaks out they shapeshift into an animal that may or may not be mountable like giant cave spiders.
any way evil nightmare biomes are if I remember toady words '1 out of 10 worlds' would have them so they are pretty rare to seek out and only happens if you're alone at night in those areas so having companions, adventuring party members, and pets will counter the effect traveling through them.