Looking back at this project, I don't think I'll continue it. But if anyone wants to try doing it, go right ahead.
How was I going to attempt to add new species? Change the weight key deformers of the model. I haven't tested anything so I don't know if you can add more deformers or if you can only edit the already existing ones.
Also, I realised that you will be limited to the anatomy of the skeleton, so species like horses aren't possible unless you don't mind it looking deformed. Maybe you can edit the animation of one of the actions to be an idle pose for the poor deformed creature (particularly stretching and squishing some bones of the skeleton so it will not be deformed for that action only). Whatever you want to do.
Have fun.
Edit:
Oooorrrr... I just got news of something. Not 100% sure if this is absolutely possible, but apparently body parts separate from the body model can have their own skeleton?
Fyi, that's a whole new ballgame for adding species. You could possibly make a whole new species model and skeleton (as long as the new skeleton has the same amount of bones, same bone names, and all of the animations that the original skeleton has). Just change the name of the exported model and skeleton to a mane or whatever else you guys are willing to sacrifice and try it out.
With this, you'll be required to do rigging, animating, UVs, textures, and masks. Not sure how you would be able to do markings though. Probably doing a preset would be the best option. You might actually need a preset anyway to hide the original model.