Thats a neat shape. I don't own any carboy's. I could be completely wrong but something about it tells me it is not old. It is way too shiny and I see no wear on the base. I hope I'm wrong for your sake but that's my 2 cents. I'll wait to see what the real experts have to say also.
Welcome to the forum! I'm afraid I agree, it's not an antique. That seam which runs around the base of the lip is only found on modern bottles. I'm not sure what it is though, it's definitely a bizarre carboy. I'm leaning towards it having started off as a standard carboy and then getting altered by an artist by heating it to a high temperature and putting it onto a mold. That would explain why the seams vanish partway down the bottle, which is not something that should happen with seams normally.