This page presents both trigonal versions of square-based fairy pieces and pieces that are purely trigonal constructs.
This piece can be classed as a Rook Slider since the individual steps are both a- and e-wazir. It can be described as a half dome alternating rider.
It takes 3 steps clockwise or anti-clockwise as though it were a dome, followed by 3 steps in the opposite direction, and then repeats the pattern.
The Hydra's movement is illustrated by Diagram 1.
to be continued…