So, I'm thinking of either making/"changing" a character to have a prosthetic tail. And by that I mean physically adding a metal tail that wasn't there before. For example, on a Floran. Would this be possible, and if so, allowed? For Apex I'd assume that it would be allowed as they already have tails. If this needs to be moved, please move it. Seriously, I have no idea where to put this. If you wanna know the reason why tail: spikes.
Not entirely sure if this is cool or not but if so... Wiring a tail into the nervous system is nothing like wiring in a replacement organ or limb. Since the brain of the organism is designed to control the parts of the body that already exist, making a tail would require making new neural pathways as well as 'reprogramming' the brain. Reprogramming is the simplest word to use here, as in reality to perform this feat one would need to be a remarkable neurologist. This could not be accomplished by one person, as it would take people with differing occupations such as neurobiologists, chemical engineers, biomedical engineers (of many types), etc. Overall seems kinda pointless I guess.
It could be possible for humans, due to our tail-bones and the fact that some babies are still born with "tails", but it'd be pretty rare to still have your "tail" after birth, as most doctors get rid of them so you can sit properly. Though having a tail would be pointless and expensive.
If it's a prosthetic for a character that lost their tail, I wouldn't see a problem with it, but I definitely agree with the others on this one if it comes to a species that doesn't have a tail normally. There are too many factors, and besides, metal isn't smart. You could use rubber / silicon with a metal skeleton to allow it to move, though, I'm thinking, if it was approved.
