The first name is a bad attempt at a topographical locative. It has several problems. First, of is unlikely. Secondly, misty is _extremely_ unlikely (though Mis Tor in Devonshire is apparently 'mist hill'), and lowlands is even less likely: we haven't seen the term in period nomenclature, and it's far too unspecific to be a useful topographic address. Thirdly, the name is much more complex than medieval examples: There are authentic bynames that include a modifying adjective, but they're much more definite and straightforward, e.g., atte Fayrehope 1332 'at the fair valley'. This is the sort of thing that we'd suggest if someone actually came to us with of the Misty Lowlands; other possibilities are I'the Medewe 1280, in le Holwe 1279, atte Hamme 1296 'at the flat, low-lying land beside a stream', and the unattested atte Dernedyngle 'at the secret or hidden deep dell or hollow' and atte Dernhole 'at the secret or hidden hollow'.
The byname of the Flowing Rivers is of the same type and has exactly the same problems. In particular, it's useless as an address because you can't possibly live by _all_ the flowing rivers. Loudwater in Berkshire was la Ludewatere 1241 'loud-water', and we might well suggest a byname like de la Ludewatere. Similarly, Henry atte Lude 1275 'at the loud (one)' lived by a loud (and therefore presumably fast-flowing) stream.
The byname of the Silver Stallion is basically an attempt at some kind of nickname, unless it's to be taken as a sign name (from an inn or the like). The latter are much rarer in English than most people realize, and most use an unmodified noun, like atte Dragon 1374, the byname of a brewer. Still, William Sevensterre 1355 'seven stars' probably took his name from a shop or inn sign, so the basic construction isn't actually impossible, though of the would need to become atte (or be omitted entirely). In medieval form the name would be atte Silverstaloun (or -stalun, or -stalon). However, stalun 'stallion' was a borrowing from French that seems not to have been particularly common until at least the 14th century, so it's not likely to have been used in an inn name. Much likelier here is the attested del Whithors 1273-74 'at the White Horse'. (When applied to a person the term stalun meant 'a begetter, a lascivious person'.)
The other interpretation of of the Silver Stallion is that it is intended to signify ownership of a silver stallion. Here, too, one would expect the more prosaic Whithors 'white horse' (also Whitehors 1331-32). There are in fact quite a few attested nicknames of just this type. Most of them didn't use a preposition. When they did, it wasn't of, but with, as in Wythehogges 1316 'with the hogs', probably a swineherd, and With the Botoun 1338 'with the button'. By our experience Wythe Whitehors is improbably complex, but at least it follows attested syntactical patterns.
Finally, the Fey Wanderer of Ealdormere, like the others, is too complex. It also uses a non-period SCA cliche, the Wanderer. Why do so many SCA folk want to stigmatize themselves as rogues, vagabonds, or petty thieves? (That's a rhetorical question; the term has a romantic connotation in our modern culture that is quite opposite its medieval implications.)
Doubtless Fey will strike most people as questionable, but there actually is a Margaret le Fey in record in 1332. Margaret le Fey de Eldermere (giving the place-name a medieval instead of Old English spelling to match the rest of the name) is a possible documentary form, but it's too complex to have been a name in the everyday sense. She might also have appeared in record as Margaret de Eldermere dictus le Fey 'called the Fairy'. Wanderer, however, is completely unattested in English, though the idea can be expressed authentically. For instance, there are bynames that were given to pilgrims, including pilgrim itself, as in Pilegrim 1185, Pelrim 1221, Peregrine 1243, and Pegrin 1275; another is seen in Cristiania la Romere 1274 'the Romer, one who had made a pilgrimage to Rome'. There are also some indirect bynames, among them Dustifot' 1221 'dusty foot', for a wanderer, especially a travelling tinker.
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