etymology - What is the origin of "-ix" as a feminine variation?

Some words are made feminine by altering the suffix to be -ix. Examples:

• dominator → dominatrix


• executor → executrix


• rector → rectrix

What is the origin of this variation?
From my 5 years of Spanish, and what little I know of Italian/French,
it doesn't seem to me to be from the Romance languages.

Answer

All of these words are loans directly from Latin, where -trix is the feminine counterpart of -tor.