The Knight of the Swan is a Continental fairy tale, typically associated with Lohengrin, a knight in Arthuriana, or Godfrey of Bouillon, a leader of the First Crusade who came from Avalon (in France, not the Isle of Avalon) and became Advocate of Jerusalem. As with the story of Melusine, the story has accreted elements of other swan-related traditions to become a full cycle of tales that covers two or three generations of a family.
In the first part, there is a swan-maid. She is discovered by a young lord or knight who finds himself lost in the woods, often after attempting to hunt a white stag or similarly fairy-associated animal. She may be in the company of one or more of her sisters but the story will make a point of the lord picking up one particular skin or a gold necklace and thus capturing his own swan-maid. This is a typical Animal Wife scenario.
In the second part, the lord and his swan-lady have children. The typical number for this is seven – six boys and one girl – probably all born at the same time. They may each have a gold chain around their neck as their mother did in the first part. The mother may or may not die. If she does, there is then an evil step-mother. If not, the children will be mistreated by their paternal grandmother who doesn’t appreciate her son marrying a woman from nowhere. The boy children will either have their gold chains taken from the necks or will be cursed into their “true” swan form. All of the children flee in a panic, fearing for their lives. Resolution is when the children are all found and returned to human form, either through the girl weaving shirts or from chains being returned. If the mother is alive, she will be freed and the true evil mastermind revealed. This is typical of fairy stories such as the Six Swans.
In the final part, the actual Swan Knight makes an appearance. The knight is accompanied everywhere by a(t least one) swan, who also draws the knight’s barge. The knight appears and rescues a lady. The two marry – or live as if wed – but there is a typical fairy taboo that the lady must never ask the knight his name or about the swan or something similar. Of course, this taboo is broken and the knight leaves, despite having already started a family. He usually tells his lady his family history – he turns out to be one of the boy children, as does his swan companion – before leaving.
Again as with Melusine, the tale may have been an adaptation of Blessed Isles‘ traditions used to explain perceived Otherworld origins or incidences of second sight or the Allergy. What’s most interesting about it is the way it appears to be of continental origin but has attached itself to Arthuriana, which began in the Blessed Isles, and to Godfrey of Bouillon, whose family became very loosely attached to the throne of England when his niece married Stephen of Blois.