Honestly, in most the cases that Ive created BTS monsters, I wanted to use something specific in an adventure, but there was no appropriate monsters/entity/creature for it. So I started by going to the formula of what I wanted and went from there.
The example I'll use will be the Scurry Scree and Scurry Talus creatures from the Rifter #46
I had a several cultists living in a "secured and gates community" in suburbia, and the player would have to raid one or two of the houses in the community. I realized I wanted "guardians" or watch dogs type of creatures looking over their homes. But as I looked over the creatures from all the BTS resources I have (which is all of it as far as I know), none of the creatures really worked for this scenario.
So I started thinking about coming up with my own creation (which became 2 by the end of them). they needed to have supernatural qualities, but they had to blend into a suburban community, hiding in in plain sight. As I thought about my on neighborhood, I recalled that one of my neighbors across the street has a things for statues in his back yard... gnomes, gargoyles, lions, he just loves statuary. And I had this thought that "what if these things were actually watching over his house?" and THAT became my inspiration in building the Scurry Talus. I made them look something like stone gargoyles in natural form, but they can also shape change, allowing them to look like whatever statue they want. I gave them some minor psionics to communicate with their masters, made them easily conjured by magic summoners, and made them tough enough to be low to middle range villains for the players to deal with.
The Scurry Scree came shortly afterwards as I wanted a creature to watch the inside of their homes as well. And as statuary isnt really put inside homes, I began to think smaller, and that got me looking at the little statuettes sitting on the book shelves behind my desk... in the end they were the inspiration to the little watch dogs that are completely out of the way of the their masters, but could still be placed in whatever rooms they were most needed to watch over. The ability to scream like a screaming frog came from my watching one incidentally on you tube a day or two before (which is my favorite attribute about it).
In an effort to make it different from the Scurry Talus (it was becoming a mini version of it), I made them so that they DO NOT fight, they merely run to their masters screaming, causing the players to chase after it to shut it up fast. The horror in their case comes from the especially loud screaming they make (people as a whole do not like loud noises), and they realization that they are alarming the very people the players are trying not to alarm.
In the end, they were what I needed, and they play tested great, so I fully fleshed out their info and submitted it for the rifter.