I think that there's no logical reason not to use -one or -body as they're not even human specific. It's not like we say "everyhuman" or "anyman". I guess it's just put in there for fun, and they try to keep it consistently like that for fun.

Though you know the older series, they had humans in those right? Did they maybe use -one/body and -pony to differentiate between them and it's just a legacy they brought forward to FiM?