mulatto

Borrowed from Portuguese and/or Spanish mulato, meaning 'of mixed breed, young mule,' itself from mulo ('mule'), from Latin mūlus. The term may allude to the mule's own hybrid origin, born of two different species.

The facts here trace only the word's borrowing into English from Iberian Portuguese and Spanish; no further shift in meaning is recorded.

It's a doublet of mulato, the Spanish/Portuguese word it was borrowed from, both descending from Latin mūlus, the source of English 'mule' itself.