Arachne is an enemy found in Final Fantasy IV: The After Years. It will only attack with Earthquake, which can be easily averted by casting either Float or by Jumping to avoid it, or moderately powerful physical attacks. It may also counter any magic attack made on it with its own physical attack. It is not a particularly difficult enemy otherwise.
Arachne is the name of a mythological Greek weaver who boasted she had skills surpassing that of Athena, the goddess of weaving. For this hubris, Athena transformed her and her descendants into spiders.