There's a Rumpelstiltskin derivative story attached to Nant Carfan, in Cwm Tafalog, not far from Llyn Coch-hwyad. The Red-Haired Brigands of Mawddwy had stolen a widow's savings and driven her cattle away, and she was in a state of great despair. An old woman - a fairy in disguise, as always - appeared at her door and offered her gold to help her out, in exchange for her baby son. However, the old crone gave the mother a chance by saying that if within 3 days she could guess her real name she'd let her off. By an amazing bit of luck, the widow was returning to her house the next day when she came across the fairy dancing around like a mad thing in the forest and singing that her name was actually Sili-go-Dwt. (I think with a name like that, I'd have kept it quiet.) The rest of the story is predictable, but a nice twist is that when the baby grew up he had the satisfaction of helping Baron Owen hang the Red-Haired Brigands.
Read about it in detail at: http://www.vwales.co.uk/Folklore/fairytales/siligodwt.htm