what is nepetas tail and how does it work?

Popular headcanon is that Equius built it for her- Since to actually move her tail, she would need to have it connected to her spine, and Equius is good with Robotics of that kind. Alternatively, though, I think her tail doesn’t move anywhere other than in the Talksprites, which are guest art, and she doesn’t have one as her Dream Self, so either it’s a Robotic Implement that wasn’t replicated on her Dream Self, or it’s just. Sewn on her green coat.