Just curious if you view all of the material in the appendices that way. Tolkien makes the reference/comparison himself, and it is not a quote from the Rohirrim nor is it explicitly presented as a mythical or legendary creation any more than the other "historical records" and fantastical creatures found in his writings. It just seems odd to me that with all of the goblins, trolls, ents, wargs, talking thrushes, balrogs, wraiths, dragons and crebain running around Tolkien's stories that one particular reference would be dismissed as implausible or legendary. Tolkien (not the Rohirrim) compared Helm to a snow-troll in the appendices, and to me it's just one more little imaginative detail like countless others. The oliphaunts were just fireside tales until Sam saw one and discovered the source of the legend.
I guess it depends on how you interpret the appendices. In the preface to Appendix A, we read that the selections contained there were taken from accounts given by various characters in Middle-earth. For example, it states that section III (Durin's Folk) was "probably derived from Gimli the Dwarf".
Based on that, I assert (and have always believed) that all of the writings in Appendix A are presented as being accounts of events, as recorded by the peoples of Middle-earth, and then abridged by the author (within the context of the fictional universe, at least
).
In fact, I have always considered the appendices to have been intended to be part of Bilbo/Frodo's work in the Red Book, not Tolkien's reference material. If you take my meaning, sir
.
Therefore, I view the account of the Long Winter as being the tale as told to Bilbo or Frodo by the Rohirrim.
As for oliphaunts, I'm not sure I see the correlation. They do appear in the narrative of the story, so they are therefore unquestionably "real" creatures in Middle-earth. Snow trolls, on the other hand, are only mentioned once, in one account of a story told in the appendices, and even then, it is not by direct reference, but rather by vague dramatic comparison (unless you believe that Helm Hammerhand was 12 feet tall
).
Also, I don't recall saying that they were implausible, only that (outside of one off-hand comment in an unrelated account) they are never mentioned anywhere. Even putting that aside, and even if (purely for the sake of the discussion) I were to concede that snow trolls did in fact walk the northern lands of Middle-earth, that doesn't change my opinion in the least. The creature was never mentioned anywhere in the narrative, either book or film, and therefore has much less appeal to me than any creature who
was so mentioned.
If someone were to offer one to me as a gift, would I accept it and display it? Of course! Will I be spending $450 to purchase one? Not gonna happen.