Looking cool so far. Not sure you'll be able to transfer the lion to the horse because the number of bones is different, so it'll mess with the vertex order probably. Same for the envelope/skin from the horse to the lion, not possible to get the same topology between the two of them, so not sure it could work. I think the best way would be to transpose them manually within ZBrush using the move tools. Probably in a day or two you can transfer all the muscle anatomy, then you'll have to recreate your Ziva setup. It's just my thoughts tho, modeler thoughts and not tech anim ones.
Btw, doing horses I suggest not to only focus on ecorche from Ellenberger's book. I was doing that before, but you ended up creating a horse that is really blank and with no real existing breed. I did this Clydesdale anatomy study that we will make available later on 3dScanStore: