The term Arachnid is from the Greek word for spider. It also refers to the mythological figure Arachne. In UO, arachnids refers to scorpions, spiders and Terathan, a race of arachnids. As such, they are vulnerable to the Item Property, Arachnid Slayer. In addition to your run-of-the-mill gargantuan scorpions and spiders, there are also several named spiders.
The complete list of arachnids is as follows: