The history of the Loathly Lady figure relative to Irish sovereignty tales (in which the lady represents the country) has been well established. I have no citations by me, as I am writing this on the bus, but I believe Frederick Madden had something to say about it in his collection of Gawain stories, and it goes on from there according to the usual patterns of late 19-early 20C myth-discovery. That is, therefore, the accepted mythical ancestor of the figure, and as hunting out mythological precedents is rather out of fashion and the genealogy is well traced I don’t believe anyone has thought to dispute it, or posit any additional ancestor.
But surely the obvious classical precedent is the god-who-comes-to-dinner? Zeus, or some other god (usually Zeus as the patron of hospitality), disguises him/herself as some ugly, poor old beggar and asks for shelter and food. Or, of course, for help crossing a river, etc. This is a test, the protagonist responds according to their moral stature, and the god suddenly sheds his unprepossessing exterior to reveal himself in all his glory to pass judgement on the protagonist, along with appropriate reward or punishment. I don’t recall offhand whether any examples of that trope appear in Ovid, but it’s entirely in keeping with the system of virtue and reward evident in the Aeneid, to consider just two of the most culturally influential classical texts extant in the Middle Ages.
The main linking device is, of course, the transformation: a disguised stranger who appeared unworthy according to the ideals of the genre – poor and old and helpless in Greek mythology, disgusting and unmannerly and often old in romance – is suddenly transformed into the epitome of those ideals. In addition:
- the transformation is in direct response to the actions of the protagonist towards the stranger,
- in entering the narrative, the stranger will, explicitly or otherwise, initiate a kind of test for the protagonist, in which their response demonstrates (and thereby establishes) their virtue,
- the stranger retains to him/herself the authority to pass judgement on the protagonist’s actions after the return transformation (rather than delegating that task to the narrator), claiming the status of moral arbiter of the narrative,
- similarly, the stranger him/herself dispenses punishment or reward.
... whanne Seint Iulian reste hym aboute midnyght al forweried [weary] and the wedyr colde and a gret froste, he herde a vois that wepte piteously and cried: 'Iulian, helpe me ouer for Goddes loue or ellis I perische for greuous colde.' And whanne he herde that voys he arose al sodenly [immediately] and passed the colde water and founde that pore creature that deied nigh for colde, toke hym up and bere hym to his hous and light the fere and dede al his diligence to warme hym. And as he myght in no wise make hym take warmthe he toke hym in his armes and bare hym to his bedde... And a litell after he that apered to be so sike and as a foule lepre stied vp shinyng into heuene, saieng to his oste: 'Iulian, oure Lorde hathe sent me to the, sendyng the to saie that he hathe receiued thi and ye bothe shull reste in oure Lorde witheinne a litell tyme' And anone he vanished awaye... (Gilte Legende, ed. Richard Hamer, EETS 2006; v. 1, 144)