The first known appearance of the term "knight-errant" was in the 14th century poem Sir Gawain and the Green Knight, where Sir Gawain arrives at the castle of Sir Bercilak de Haudesert after long journeys, and Sir Bercilak goes to welcome the "knygt erraunt.
Many knights-errant fit the ideal of the "knight in shining armor". A knight-errant performed all his deeds in the name of a lady, and invoked her name before performing an exploit. Such a knight might well be outside the structure of feudalism, wandering solely to perform noble exploits (and perhaps to find a lord to give his service to), but might also be in service to a king or lord, traveling either in pursuit of a specific duty that his overlord charged him with, or to put down evildoers in general. This quest sends a knight on adventures much like the ones of a knight in search of them, as he happens on the same marvels; in The Faerie Queen, St. George is sent to rescue Una's parents' kingdom from a dragon, and Guyon has no such quest, but both knights encounter perils and adventures.
In the romances, his adventures frequently included greater foes than other knights, including giants, enchantresses, or dragons. They may also gain help that is out of ordinary; Sir Ywain assisted a lion against a serpent, and was thereafter accompanied by it, becoming the Knight of the Lion. Other knights-errant have been assisted by wild men of the woods, as in Valentine and Orson, or, like Guillaume de Palerme, by wolves that were, in fact, enchanted princes.
Youxia, or "Chinese knights-errant", traveled solely protecting common folk from oppressive regimes enacted by courtly officials. Unlike their European counterpart, they did not come from any social caste and were anything from soldiers to poets. A popular literary tradition arose during the Tang Dynasty which centered on Negrito-slaves who used supernatural physical abilities to save kidnapped damsels-in-distress and to swim to the bottom of raging rivers to retrieve treasures for their Feudal Lords.
The cowboy of the American Western genre can in many ways be considered a modern successor to the Knight-errant. Like the Knight-errant of Medieval romance (and not necessarily like the actual cowboy of 19th Century American society), the cowboy of Western novels and films wanders from place to place on his horse, bound only by his innate code of honour, and often performs noble deeds or saves a damsel in distress (though unlike the Knight-errant, he usually does not call such deeds by these names).