Akira: "I can answer that one! In Japanese folklore, Yokai are supernatural entities or monsters. Jorogumo, the symbiote, is of alien origin, and could be considered a supernatural entity... We name ourselves after Yokai or other Cryptids. For example, Osaka's hero identity, Daiouika, is Japanese for Giant Squid, or Kraken. The only exception is Ultron, as he doesn't have a civilian identity... yet. We're still working on that.
"Regardless, we serve as Sol City's hidden protectors, so we call ourselves the Yokai to show that."