Fulvous has never been a common word, but you are much more likely to encounter it in texts from the 19th century than in texts from the decades since - unless, that is, you care about ducks. In that case, you might know about a kind of whistling duck called the fulvous tree duck, which is a brownish duck with long legs and a long neck that has an unusual world distribution. It lives in isolated populations in North America, South America, India, and Africa - remarkably without geographic variation. But back to "fulvous": it shares a meaning with its direct ancestor, the Latin word fulvus, and "fulvus" itself is believed to possibly share an ancestor with "flavus," Latin for yellow.
