Rú is a Quenya name for the Drúedain. The Drúedain called their own kind Drughu, a name which the Elves rendered into their own languages and intonations. In Sindarin it became Drû, from which the name Drúedain has been derived. In Quenya, Drughu became Rú. When they were recognised by the Elves as belonging to the race of Men, and became their allies in the war against Morgoth, they acquired the suffix atani, so Quenya Rúatani is exactly equivalent to Sindarin Drúedain.