One, Many, Gender change?!

Question: Which word is singular...

That when another letter is added to the end, becomes plural...

And when another one is added to the end of the second word, changes gender?

10pts to FIRST person to get it.

---

---

Answers: Which word is singular...

That when another letter is added to the end, becomes plural...

And when another one is added to the end of the second word, changes gender?

10pts to FIRST person to get it.

---

---

Prince
Princes
Princess

Tough one

---

---

---