A static (i.e. rigid) tail can be added as an ornament. E.g. the tail on the bunny costume in my tutorial on ornaments.
If you want a moving tail, then you'd have to add bones to the skeleton, and re-make all the animations to include bone movement. That's not quite as nasty as it sounds, as you can copy all the movement for the other bones over from the no-tail ones.
When I made the Lactri Spawn for Xaa, I was able to copy the spider leg animations and the upper body animations over from the parent files, as the cycle lengths matched well enough. If there's a creature with a tail you can steal skeleton and animations from, it might not be too hard.
A static (i.e. rigid) tail can be added as an ornament. E.g. the tail on the bunny costume in my tutorial on ornaments.
If you want a moving tail, then you'd have to add bones to the skeleton, and re-make all the animations to include bone movement. That's not quite as nasty as it sounds, as you can copy all the movement for the other bones over from the no-tail ones.
When I made the Lactri Spawn for Xaa, I was able to copy the spider leg animations and the upper body animations over from the parent files, as the cycle lengths matched well enough. If there's a creature with a tail you can steal skeleton and animations from, it might not be too hard.