Animatronics is tricky. It requires mechanical engineering in addition to the software and electrical engineering of most of our projects. Normally when I talk with cosplayers just getting started with electronics, I point them toward a light or sound project, to limit the number of variables.
This project reduces animatronics to its simplest case. Mechanical tails are complex stuff, often involving armatures for bones and cable mechanisms to simulate muscles and tendons. Rather that fighting against gravity, we’ll instead make it our ally. Thinking of the tail as a pendulum, a single small servo and a little math is all it takes.