The "Boo Men" were actually one of my oldest, pre-Mortasheen concepts, but almost none of the originals survived into their Mortasheen incarnation. They were named after an opening chapter in the "Scary Stories to Tell in the Dark" book series, which in turn was said to be an obscure, older way of saying "Boogieman."
The original idea of the Boo Men, from my long-defunct horror/fantasy/scifi setting, was simply a band of elite monster assassins who each had a different signature killing method, which was also their name. This HAS survived into the Mortasheen version, though as a rule, Mortasheen's Boo Men are humanoid so that they don't just feel like any other monster. My original Boo Men included a few nonhumanoids just to really play with the "men" moniker, one of them even being nothing but a faceless, stone block and another a virus.
Braek is based obviously on Ed Roth's artwork, most famous for Rat Fink and other scuzzy monsters who were intertwined with hot rod culture. Braek is similarly obsessed with speed, and basically a "biker," but is also her own bike. She was also an attempt to do a female humanoid monster that wasn't visually coded as feminine in any way, though I guess she does have kind of an hourglass waist.
I always liked the look of those biker helmets with the spike on top, but they have some negative cultural associations and I already used them anyway for the War Bird line many years ago, so I turned it into a long lance-like horn she can use as a weapon. It also becomes her "nose" when she flips the helmet down over her eyes.
I really want to redo the look of her tire someday, which I think just doesn't look good at all.