Furby is a common WoW slang term for any type of Furbolg. More specifically it is often used to refer to either the Timbermaw Hold furbolg faction or the Deadwood and Winterfall furbolgs that must be slain to gain reputation with the faction.
Furby was also the name of a popular interactive children's toy released in the late 1990's. The Furby was the target of massive consumer demand during the holiday season immediately following its release, dubbed the "must-have" toy for that particular year.
WoW Furbolgs bear a vague resemblance to the Furby toy, despite being derived from a completely different (ursine) lore base.