"Legolas was the son of Thranduil, King of the Woodland Realm of Northern Mirkwood, who appears as "the Elvenking" in The Hobbit. Thranduil ruled over the Silvan Elves or "Wood-elves" of Mirkwood." -Wikipedia
I would say it is entirely resonable for Legolas to appear in this movie. Never read Hobbit, but that is what fast googling got me.