If you only include the OT then he didn’t have a first or last name, so who’s to say he was even a human species. Probably a human-like species. He might have a tail under that robe.
That’s true but if Stat Wars only consisted of the OT I wouldn’t want take a look under the robes to find out.
Maybe his species just has yellow eyes, like cats.
He’s human but he’s so evil that his evilness can circumvent biology. He’s just that evil.
Maybe he’s a she, and maybe she’s a Changeling.
The same thing I said to MathUser applies. If The Emperor was a changing I would be baffled to believe he would ever show his true form. If I were a changling I would probably turn into a Ithorian most of the time.