Guingamor

Guingamor is an anonymous medieval lai about a knight who leaves the court of his uncle, a king, because the queen has sent him off to hunt for a white boar. By offering a reward for the boar's head, she hopes to get rid of the protagonist Guingamor, who has refused her sexual advances.
Guingamor crosses a river and passes into a mystical kingdom. Returning with the boar's head after what seems to him like three days, he encounters a common charcoal-maker, who tells him that many years have passed since the king's faithful nephew never returned from a hunt for the white boar. Guingamor's return is triumphant and he is immortalized in a lai. 
The story was once presumed to have been written by Marie de France, but is now considered anonymous. However, it draws on Marie's "Lanval", and the anonymous "Graelent":