"Old fat spider spinning in a tree!
Old fat spider can’t see me!
Won't you stop,
Stop your spinning and look for me!"
Ettercaps are a species of spidery humanoid monsters from the various worlds of Dungeons & Dragons. Whilst they share this niche with at least two other races — the chitine and the aranea — the ettercap has surpassed them by actually managing to stick in the general public awareness. They are considered an Aberration race, but despite this have an almost — but not quite — druidic outlook. They seek to protect and foster spiders, herding them and breeding them, and whilst they do warp the forests and ruins they dwell in until they become twisted, creepy mockeries of their former selves, it's all to give their beloved spiders more places to feel at home.
In general, ettercaps are basically somewhat bigger, smarter, creepier spiders, with no culture or society outside of herding spiders and hunting like a giant spider.
The ettercap goes all the way back to the Fiend Folio of Advanced Dungeons & Dragons 1e. Here, they were presented more like hairy goblinoids with vaguely spider-like claws and an affinity for taming giant spiders. They were then upgraded to 2nd edition in the Monstrous Compendium Volume 2, before hitting the big leagues by getting into the famous 1993 Monstrous Manual; this change made them more spidery and less goblin-like, with distorted features that were humanoid, but also somewhat spider-like.
They fully morphed into their now-iconic humanoid spider format in 3rd edition, and have appeared in every Monster Manual from that edition onwards. They received an "Ecology of the Ettercap" article in Dragon Magazine #343, which presented the theory that they are the mutated descendants of a deranged cabal of spider-worshipping druids, corrupted by generations of fleshcrafting and unholy interbreeding with spiders that came about after they were taken over by some spidery Demon Prince. Surprisingly, no, it wasn't Lolth.
In 4th edition, the ettercaps really got their shit together. They were considered humanoids, and started hanging out with other humanoids; they even started using weapons and armor. The fashion sense of the ettercaps evolved too — they shed their drab, goth-inspired purple skin in favor of a faux-chitinous beige, and two extra arms. They probably even moved out of their mother's basements and got jobs, but this wasn't mentioned, in a move doubtless taken to avoid upsetting 4th edition's key demographic.
Ettercaps in 5th edition regressed completely back to the two-armed, hunched purple sulks familiar to their depiction in 3rd edition (although they moved from Aberration to Monstrosity, which is the new catch-all category).