Heh, the dictionary definition that stuck with me however wasn't the mythological one but "a thin watery or blood-tinged discharge" (aka "pus"). Neither one is something I'd like my eyes compared to. I mean granted, it was Harry Potter thinking about Draco Malfoy, and maybe he was keeping in the tradition of the icky love poems he himself got (describing his eyes as "as green as a fresh pickled toad" after all), but I doubt that was the effect the author was going for. Besides, at least pickled toad is apparently green, whereas "ichor" doesn't sound like anything eyes would be like unless the guy was hungover with blood-shot eyes or something.