Tarantula Mortis and Burning Araneida - Sacred
While this game has a slew of more generic spider enemies, the Tarantula Mortis is both the largest and most visually interesting. It has a distinct head, unlike real spiders, which has a very soft and fleshy look to it that makes it feel slightly unnatural. Neat.
Despite how massive and powerful it is, one got captured and put in a zoo. Perhaps it was captured as a hatchling. It could probably easily escape by climbing over the walls, if it wanted to, so maybe it's content with being fed.
It also has a red, flaming variant called the Burning Araneida, which is classified as a demon. Note the screaming skull pattern. It's quite rare too, only appearing in the basement of one dungeon.