This knight known as Sir Hokus of Pokes due to his spending several centuries imprisoned in the dull, sleepy city of Pokes. It's there that Dorothy and the Cowardly Lion find him during the events of The Royal Book of Oz, and help him to escape at long last. He's an impulsive old man, always wanting to go on a quest or slay a monster, and often looking at things through the lens of medieval romance. Sort of like Don Quixote, actually. Unlike seventeenth-century Spain, however, Oz actually has giants and dragons. They're just not quite what the good knight expects.
Sir Hokus is quickly accepted as part of Ozma's court, and makes at least token appearances in most of the first few Thompson Oz books. There are several references to his being seven centuries old, and strong hints that he originated in King Arthur's England. Arthur is generally thought to have lived in the sixth century or so if he existed at all, but this was some time prior to the era of knights in armor. Over the years, the Arthurian legend came to incorporate a lot of anachronistic material, and the castle with the Round Table and all that is probably much closer to the thirteenth century or so, which would make it about right for Hokus' seven centuries. It doesn't really matter in the end, however, as Yellow Knight reveals that the knight isn't from Merrie Olde England after all, but rather from...well, I don't want to spoil the ending. If you want to know, read the book. Or look up spoilers elsewhere online. Whatever.
During the course of his adventure in this book, the knight comes across the dreary kingdom of Marshland (which, despite its name, is technically a swamp, as it has trees), where the hideous Queen Marcia tries to force him into marriage. The giant turtle Ploppa initially takes Hokus to see the queen, then helps him to escape when she locks him up. Like a lot of Thompson's Ozian animals, Ploppa is very friendly and devoted, and is upset that he can't continue to travel with the good knight. As the turtle himself puts it, "I must have mud and you must have adventure. Oh, why cannot people who like each other like the same things?"
Oh, and happy birthday to fellow Oz fan ozma914!