Oh, the random tidbits the Internet provides:
And a moment’s work with Google supports it, giving a reference:
This page reports that, according to Humphrey Carpenter’s biography of Tolkien, he went to a New Year’s party (not, apparently, a costume party) dressed as a polar bear; a more recent biography by Neil Heims recounts the same incident, stating that Lewis joined Tolkien, both at the party and in the choice of costume.
The other “five fascinating facts” weren’t news to me, but I have to admit this one was.