Could it be "usted,* which is the formal form of "tú"? The plural of "usted" is "ustedes," so maybe there was a mixup because they're similar..?
Another form is the "vos," used in countries such as Argentina, Honduras, and some regions in a couple of other countries. The fact that it ends in "s" may lead some people to think that it's plural, but it isn't.
Both usted and vos are conjugated differently from tú.