The idea of ‘Robin Hood’–one who steals from the rich and gives to the poor–or even someone who is on the side of the weak and downtrodden against the unjust ruler, is very old.  One of my favorite books, Sherwood by Parke Godwin, sets the story in the time of the Norman conquest, making Robin a Saxon thane.  Sadly, it’s out of print, but you can get it used from Amazon.com:  http://www.amazon.com/gp/offer-listing/0688052649/ref=dp_olp_used?ie=UTF8&qid=1326154880&sr=1-5&condition=used

We have other choices for Robin Hood that are set in Wales:

Bran ap Brychan:  Stephen Lawhead’s King Raven series focuses on this possible hero.  Like Parke Godwin’s, Sherwood, Lawhead places his Robin Hood at the time of the Norman conquest–though of Wales, not England.  Bran, the “heir to the throne Elfael, has abandoned his father’s kingdom and fled to the greenwood. There, in the primeval forest of the Welsh borders, danger surrounds him—for this woodland is a living, breathing entity with mysterious powers and secrets, and Bran must find a way to make it his own if he is to survive.”  http://en.wikipedia.org/wiki/King_Raven_Trilogy

I have not been able to find any evidence that Bran ap Brychan was a real person.

Dafydd ap Siencyn:  During the War of the Roses (1468), Dafydd declared himself for the Lancastrian cause and brought his men into the fight dressed all in green (thus leading to the Robin Hood association).  “A native of the Conwy Valley and kinsmen of the Wynn family, Siencyn used a rocky outcrop in the Gwydir forest as his base, the cave where he lived known locally as Carreg Y Gwalch.  Dafydd ap Siencyn led a band of men to Denbigh, the main Yorkist stronghold in North Wales, and burned the entire garrison. The king was said to be furious and gave the order to William Herbert, Earl of Pembroke to “lay waste to the whole Conwy Valley”.”  http://www.llanrwst.net/history.asp?contentcatid=2&contentid=15

“He received a pardon in 1468, and later he was appointed Constable of Conwy Castle, after killing his predecessor. Dafydd is said to have died of wounds received in a brawl, and he composed two poems on his deathbed.”  http://cy.wikipedia.org/wiki/Dafydd_ap_Siencyn

Twm Sion Cati:  “Although commonly referred to as ‘The Welsh Robin Hood, Twm Siôn Cati (c1530-1609) deserves better respect. He was of noble blood, a poet and a heraldic bard of renown. Many of the escapades attributed to him probably flowed from the imagination of various novelists.
According to recent research, it is possible that he hid in the famous cave at Rhandir-mwyn to escape religious persecution rather than to avoid the wrath of people who had been tricked by him.”  http://www.rhandirmwyn.net/twmsioncati/story.html

For once, I don’t have a bias towards the Welsh version, as one of these characters are particularly heroic and there’s nothing inherent in the legend that implies he was Welsh.  But it’s fun to speculate.