I was thwarted in my attempt to give drow druids a spider 'animal companion.' It was my favorite monster of the Underdark Animals article, and the one they cut out. Maybe you could find a way to get one in there. If not for me, do it for the children. Children love drow druids with giant pet spiders.
Personally, when I was a kid, I was attacked by an anthill at the age of 6, which led me to never stand still while outdoors until middle school. I would always shift from foot to foot, assuming ants would not be able to climb onto me that way. Does D&D have stats for the irritation of common things like ants?
And once when I was in 4th grade I was stung in the middle of my forehead by a wasp. I always took great glee in spraying those buggers with high-powered streams of RAID. What about a monster that gets payback on people, spraying acid or poison in high-powered jets?