In Mojave and NCR turf, they are almost unknown. However, they are far from being confined to Arizona and alike. You can swindle mentions of them in quite a few places, under different names and superstitions. Some believe them to be something ancient that was disturbed by echoes of the Great War. Others fairly certain they are an offspring of it, mutations, perhaps even a very strange and rare strain of 'feralism' in ghoulified humans (Or birds. Or insects. Or a product of... both? Maybe they are... a pencil?). Some deem them quite intelligent, others would say their brain capacity doesn't go further than that of a little child. Lustful morons from settlements close to their alleged domains would claim they laid with them, even if most fables that touch such a subject describe belowers as hermaphrodites that would only grow apropriate organs for shivering mating seasons a human would not survive (And lose those by the end of it like a snake a one too extra skinflake).
They say they are nothing more but mindless monsters, yet there are plenty of stories about Stridors capable of operating human machinery, weaponry including guns, crafting things of their own, and... deep in depths, there are whole cities of underdark, built by them... At least that's what the barkeep told you while filling your cup. Would you doubt such a reliable source as that?

But there's one thing all 'invisible killers' share, in every tale. They don't necessarily like you. In those rare cases where Stridors will engage in some kind of contact with humans, they would usually demand a high price in return. If you meet a stridor on a crossroad and ask him where's left or right, be ready he might demand your whole hand for the answer. Literally.

Tribe From Below are perfectly capable of imitating human speech (Which, when coming out of their mouths, resembles a dual song of a cricket and an angered snake), however, their own language and ways of communicating, vastly different from your common wastelander. Let’s just say that you hear a strange  screeching frog-on-a-mosquito quacking in the shadows, or owlish echoes from the pores in the ground around you, chances are you being naughty and eavesdropping on someone’s private conversation.

They worships Mazoga, the mother of an All-Being in the religion they share with War Horde (It is also can be presumed that it's War Horde, the only human tribe in a 'continuously friendly relations' with Belowers, who brought this homebrewed cult to them, not reverse), which sincerely certain the world was created by a devilish demiurge and hence the natural state of everything is evil, while good is unnatural if not downright heretical and destructive. Interestingly enough, 'what is evil' is not a defined subject amongst them, and even rumoured to bring inner conflicts that splat out splinter factions. But in the end, if Stridor himself is nothing but a rumor, then those are rumors about rumors.

Besides having natural cloaking abilities similar to nightstalkers and a scary level of durability, the alphas of their gene-caste society are also extremely adoptable, like if they force evolution on a whim. Wherever they go, in a generation they'll feel at home, except if it involves the sun and day life. But even that is not always your ultimate hiding place, since creatures that vaguely resemble Stridor alphas inhabit Zion, and stroll in a day thinly often.
At the same time, because their evolutionary stew is always boiling and always one second away from pouring over the soup bucket's walls, they are extremely prone to genetical diseases and deformaties, so much in fact that almost every Stridor 'suffers' from one or another (those that are born normal are the 'weird'). Most usual and visually obvious peculiarity of this sort are sets of underdeveloped, extra fingers on each hand.

Are Stridors a threat for You, the abover? Well "is the ground you walk upon is your enemy", here's the question you should ask yourself.

But if you come uninvited, lacking any tokens and passage rites, then expect their cold welcome.