A very unusual appendage.

>>Many times, when new species are discovered, the characteristics that set them apart can only be appreciated by scientists. That isn’t the case with the Ceratogyrus attonitifer tarantula, however – the thing has a long, soft horn protruding out of its back, and nobody knows why it’s there.<<
https://newatlas.com/new-tarantula-horn-back/58446/