Note: This will all be revised and rewritten in a more proper manner.
I just made Flesh Spiders up right now but hear me out
they're basically spiders but they're pretty big (about waist high when they're alking around) and they have bones and flesh not dissimilar to mammalian animals
they're covered in short but soft duvet-like hair, a few millimeters long, just enough to give them a softer texture
you wouldn't want to touch them because they're nasty and lubricated by a substance they secrete which helps them both mask their scent and slip against rock more easily (for going through tunnels)
if you cleaned them maybe they'd be nice
but they'd be more like long rectangles since these fucks have thin and long limbs
as the name implies they're mostly from the underdark. For creatures of the underdark they are unusually large (usually, that environment favorises smaller fauna that can slip through small gaps and holes)
That's where the long, thin and articulated limbs come in, along with lubrication. Since the core part of their body is pretty small, they are capable of folding up these limbs very close and in a form of crawling motion. These limbs enjoy a lot of freedom and are capable of gripping the surfaces in almost all directions to help with locomotion, while the areas that shouldn't grip are lubricated enough to just slide against the rock.
This makes them able to thrive in the underdark despite being unusually large: waist height when walking (and about the full height of a man in width and length) but capable of retracting into a smaller form about the size of two cinderblocks set side to side.
Flamethrowers work pretty well: the lubricant (called "spider sap" by the locals) is very flammable. Unluckily for us, being attracted to warmth, they rush towards the source of the fire: their fried brains don't seem to understand the difference between simple warmth and burning napalm. It is recommended to put some distance between you and the spider until it burns out.
"Yeah you kinda go crazy when you suddenly catch fire."
they're drawn to warmth because they live in caves where it's pretty cold. In their last moments that goes wack tho.
"What's really disturbing is mamalian spiders
"
Yeah, that's supposed to be the fucked up part. Plus when they're crawling through tunnels you can hear the creaking and snapping of their cartilage as their bodies contort to squeeze through.
"Do they produce milk for their young?
 Do they give live birth?
 Gives me conniptions."
It really is flesh and bone. I'd say they give birth similar to a platypus.

"Ok so what parts of their bodies can be used for making stuff? Like what qualities make their leg bones good to use for rifle stocks?"
The bones are sturdy, relatively light and are good at absorbing shocks because of they are a little bit more flexible than regular bone. They are also pretty long so you have a lot to work with.
The flesh is poisonous raw but is fine to eat once cooked, but it does have a weird aftertaste, almost rancid. Seasoned it's pretty good though, and it's sometimes sold by rare Drow merchants on the surface. It is quite popular with halflings for some reason.
The cartilage has been used for primitive springs but it's hard to shape: it falls apart if you fuck too much with it. Sometimes it's boiled along with the bones for broth, after which they become malleable until they get cold again.
The eyes and the organs are laced with poison which are not neutralized by heat. Because of this, the core of the body is usually left untouched if you're there for food. It's still valuable though, some merchants will give you good money for it. One of the reasons for this is that small crystals in their eyes can be extracted and used for their unusual optical properties such as light intensification, which allows the spiders to see better in the darkness of caves. Pretty unconventional for biological night vision, but very practical for those than know how to extract them.
The skin is somewhat gooey and slippery because of the "spider sap". Once cleaned however, it is soft and pleasant to the touch. Some turn it into leather and some use it like you'd use fur once the flesh side has been cleaned and scraped. It's relatively sturdy and retains some of its elasticity even when dried or turned to leather.
Gun furniture like pistol grips and handguards have been made from the bone, and sometimes even stocks when the section near the joints is wide enough. Some say that those stocks help dampen recoil but that's mostly disregarded as a fuddlore kind of rumor. Almost everyone agrees that it's sturdy and that it looks nice though.

Flesh Spiders are dangerous to hunt for both novices and experienced hunters, but they are very useful. They are not extremely valuable but are a welcome sight for merchants (when they are dead, obviously). You'll usually get a good price for them. Whether that price is worth making your way though the horrors of the underdark and being stalked by horrific creatures such as these is up to you.