Idr is the name of a creature created by Idarran of Ulivo, an insectoid like creature made by magical mutation and various genetic experiments. One of the last ones was killed 1373 by Geralt in Magpie Forest.
Idrs had a long, flat, gray, segmented body with many limbs to help it move. It was also covered in small hair-like feelers and bristles to detect far off vibrations, allowing it to hone in on possible prey. These were so sensitive that it could detect individual targets out of a group and allowed it to plan its attack accordingly.
For attacking purposes, an idr had razor-sharp mandibles as well as pincers.
The creatures were known to be particularly vicious, hunting to kill for pleasure rather than for survival. It was also very intelligent and could pull tricks even on witchers, feigning to go for one target but then going for the real one. Combined with it's incredible agility made it a formidable creature to come across.