[Legolas]

Tolkien says...

Fafnir in the late Norse versions of the Sigurd-story is better; and Smaug and his conversation obviously is in debt there.

This legendarium ends with a vision of the end of the world, its breaking and remaking, and the recovery of the Silmarilli and the 'light before the Sun' – after a final battle which owes, I suppose, more to the Norse vision of Ragnarök than to anything else, though it is not much like it.

The language of Dale and the Long Lake would, if it appeared, be represented as more or less Scandinavian in character; but it is only represented by a few names, especially those of the Dwarves that came from that region. These are all Old Norse Dwarf-names.

I remember much of this process – the influence of memory of names or words already known, or of 'echoes' in the linguistic memory, and few have been unconscious. Thus the names of the Dwarves in The Hobbit (and additions in the L.R.) are derived from the lists in Völuspá of the names of dvergar; but this is no key to the dwarf-legends in The L.R. The 'dwarves' of my legends are far nearer to the dwarfs of Germanic [legends] than are the Elves, but still in many ways very different from them. The legends of their dealings with Elves (and Men) in The Silmarillion, and in The L.R., and of the Orc-dwarf wars have no counterpart known to me. In Völuspá, Eikinskjaldi rendered Oakenshield is a separate name, not a nickname; and the use of the name as a surname and the legend of its origin will not be found in Norse. Gandalfr is a dwarf-name in Völuspá!

Letter 279 gives the most information on Tolkien's borrowings. Plagiarism? Tolkien didn't claim it to be all his own work:

A few years ago I was visited in Oxford by a man whose name I have forgotten (though I believe he was well-known). He had been much struck by the curious way in which many old pictures seemed to him to have been designed to illustrate The Lord of the Rings long before its time. He brought one or two reproductions. I think he wanted at first simply to discover whether my imagination had fed on pictures, as it clearly had been by certain kinds of literature and languages. When it became obvious that, unless I was a liar, I had never seen the pictures before and was not well acquainted with pictorial An, he fell silent. I became aware that he was looking fixedly at me. Suddenly he said: 'Of course you don't suppose, do you, that you wrote all that book yourself?'
Pure Gandalf! I was too well acquainted with G. to expose myself rashly, or to ask what he meant. I think I said: 'No, I don't suppose so any longer.' I have never since been able to suppose so. An alarming conclusion for an old philologist to draw concerning his private amusement. But not one that should puff any one up who considers the imperfections of 'chosen instruments', and indeed what sometimes seems their lamentable unfitness for the purpose.