Kantasian Bog Spider

The Bullywug cavalry is one of the most terrifying sights imaginable. A host of large toad-like people with slimy pustulent skin wielding spears and slings atop huge hairy eight-legged mounts, mandibles clicking and drooling in excitement. The Kantasian Bog Spider, as Faerunian zoologists have labelled it, is what takes the Bullywugs in The Frog Bog from a competent local group of skirmishers to a swift and nightmarish cavalry, capable of feats no other surface force could boast.
When the 'horses' charging at you suddenly scurry up the side of a wall or mountainside and come at you sideways, or simply scale a vertical cliff, or leap between treetops over narrow strands of web, there is no defence. The one thing you could count on with giant spiders was always that they wouldn't think overly tactically and would occasionally run away. Trained spiders of war don't retreat, however, and are led to perform incredible manoeuvres beyond any expectation.
Bullywugs have spent years training these massive arachnids from their eggsacs. The story goes that one hunter saw the potential in taming them, and led a raid on a nest of the spiders, dragging a sack full of eggs back to the camp for training. One can scarcely imagine (nor would one want to) the birth, the scurrying, the chaos, the pens, the trials, the setbacks, the teething, the biting, the growth spurts, the joyful bonding of frog and spider, the rigorous military training, the webbing target practice or the ultimate cohesion reached as these spiders grew up and up. It is something that no one should see, and that fact that it exists should upset everyone.
A Bog Spider in its natural habitat will make its home in caves with a pack of its fellows. They enjoy leaving traps of webs around the surrounding swamp, which they then check, repair and improve each morning. Though they are very large, their steps are delicate and move across branches in the treetops hardly disturbing a leaf. Thick hair coats their bodies so that leeches slide off them if they have the opportunity to swim, and also provides some dark grey and brown camouflage for hiding in the shadows. As tamed beasts, they are often directed to do much the same, so that the trees near to the Bullywug camp are thick with webs and traps to catch all manner of creature to feed both the spiders and the rest of the village.
In the words of one scholar from Neverwinter who came to see what these creatures were: "I'm out. No seriously, where's the portal I want to go home. Did you see those things? I would like to put literally uncounted miles of distance between me and those monstrosities."