Semi-simple. You'll want to convert the skeleton model into a format where you can modify via Notepad however
Firstly, you'll need to add a Tail node to the skeleton. The easiest way to do this will to be going into the human model and copying it from there, given it'll be at the same height. Another way would to be to go into say, the lizardman, and copy it from there
The second thing will to change the animation super at the top of the skeleton from a_ba (Standard animations) to the mounted animations, but I don't remember what those are off the top of my head
The "tail"dummy node - which can be copied from the rootdummy - needs to be positioned at x0/y0/z0, its orientation set to 0/0/0, and parented to the aurorabase. Next, set the animation super to h_ba. In appearance.2da you'll need to set the "ModelType" to "FT."