There is one Cthulhu it's safe to invoke; Sollasina cthulhu, obviously named after H.P. Lovecraft's tentacled monster, a 430 million-year-old fossil that
has been identified as a new species related to living sea cucumbers.
The new cthulhu, Sollasina, was an echinoderm (the group that includes sea urchins, sea cucumbers, and sea stars) that had 45 tentacle-like tube feet, which it used to crawl along the ocean floor and capture food. The creature was small, about the size of a large spider. It was found in the Herefordshire Lagerstätte in the United Kingdom, a site that has proven to be a trove of fossilized ancient sea animals, and included soft tissue preservation.