Oh yeah, Warp Spiders are the ancient ones. I was perhaps thinking about the Fire Dragons which are also ageing, iirc.
It's honestly really weird how haphazard the Eldar range is, considering that it's basically one of the core 40k factions, and virtually ALL its units are "iconic" in the sense that they inhabit distinct visual design niches and gameplay tactical niches. It's not like one of the Eldar units is another rando Space Marine Captain with a slightly different loadout or a IG tank with an incredibly questionably useability.