Horse

Four hundred years ago, surgeons transplanted the first heart.
If they saw what people can do now with nanobots and universal tissue printing, their jaws would drop and never come back up.
And they don’t even need a surgical team. Just schedule some time in a bodymod tank, and your nanobots can pretty much work up anything you can imagine.
Just make sure you run morphology backups before you get fancy. My cousin Jake went centaur and he had to wait a week while Central Archives dug up his file.
I swear, his apartment still reeks of rancid horse poop.