Yesterday I made this video, featuring the first prototype (printed in transparent green PLA). As you can see, the electronics and the tail's "skeleton" are fully functional now.

An accelerometer senses the user's movements, and an Arduino mini pro translates it into commands for the two servos that move the tail. Right now, there's a delay of 0.5 seconds between detection and the servo's movement - This is intentional, and in the final version the delay will be replaced by a smooth, more natural movement.