Did King Arthur slay any dragons?

In the legend, King Arthur did not slay any dragons. He was far too busy trying to unite all of England under the rule of law, and later managing the knights in their search for the Grail.

I strongly recommend the book The Once and Future King by T. H. White; it looks large, but it is extremely readable and quite funny in places. In that version of the legend, Arthur and his stepbrother Kay have an adventure with Robin Wood in which they free one of Robin's men from the faerie folk; the group are set upon by a wyvern after the escape, but my recollection is that it is Kay who kills the wyvern, rather than Arthur.