Who needs sleep when you could have nightmares?
Harpoon spiders are delightful Large-sized, 5 RHD aberrations. Their stats are okay, though strength is on the low side.
Their natural weapons are a poisonous (dexterity damage, medium DC) bite that deals unimpressive damage otherwise, and two ranged fangs that can harpoon other creatures. Harpooning is a mess of an ability, but it amounts to giving every fang attack a chance to drag a victim back to the spider, where it eats a free bite attack (which is probably the one from the spider's Improved Trip) and is presumably left prone. That's decent, though harpooning essentially shuts itself down (can't make ranged attacks that well if there's foes nearby) makes it way harder to use.
The spider's final abilities are Web Movement and Spines. The former does exactly what you'd expect, the latter makes attackers striking it take slight piercing damage.
In the end, I don't think the lost BAB, self-negating nature of Harpooning, and bad HD make up for the abilities gained. -0 LA.
Harpoon Spider, Dread
Four more HD aren't worth the size increase. -0 LA.