Fillian scouts ahead of the party, leading his horse by the reins. He concentrates on tracks, bent branches and such and so he misses the shadow swaying in the breeze above. A loud rustling is the first sign that something's wrong and before he can react, his horse is on its side, kicking and screaming, caught in a net. A few strands catch on Fillian's clothes and armour and he becomes entangled as well. His screams bring the party running to help him and they find him struggling to free himself from the net, while he and the horse are hoisted up into the air by a long-legged spider busy wrapping the horse in layers of silk.
Net casting spiders are also known as ogre-faced spiders. Why, you ask?
Photo by Robert Whyte
How can this not turn up in some fantasy campaign with that name?
They are unusual because of their hunting technique. The spider will build a small net from fuzzy, very elastic silk that is not sticky, but will attach itself to prey like velcro. Or rather like some fluffy wool - if you have ever handled wool with hands that were even a bit roughed up, you know what I'm talking about.
It then holds the net between its first four legs and will wait for an insect to pass by. Because of its huge eyes, the spider has excellent night vision. The net is cast over the prey and then wrapped up quickly.
photo by Wikki Will
For roleplaying purposes, it would be very hard to cut through the net because it is so extremely elastic, it can easily stretch to twice its size and more. It should burn nicely, but I don't know if that is the best course of action to free a party member caught in the web. During the day, the spider probably doesn't have such excellent vision, but at night it can spot prey even by starlight.
David Attenborough once again. The Net casting spider is shown from 2:00 onwards, making its web and catching a cricket.
Edit: I just came across another video with excellent slow motion of the catch and a very excited scientist. Here you can see the cricket touching a trip wire before the spider pounces on it.
Sources: Science Spider Friday
a list of Strange Beasts