Something that's fabled is very well-known through stories or word-of-mouth, especially for its excellence or rarity. It's worth planning an all-day visit to a museum with a fabled art collection.
Some fabled things are legendary by virtue of having actual legends, fables, or myths mention them. You might read about the fabled Greek gods, for example. Other things or people are fabled because you've heard them mentioned in conversation — their reputation proceeds them, like your dad's fabled chocolate chip cookies. Coined around 1600, fabled first meant "invented" or "unreal," from the Latin root fabula, "story."