Hello, Shawtay!
Even though both NPC's (Marcus and Waverider) tell us about Klabautermann and a book says so, I personally don't think they truly exist.
Klabautermann is an ancient legend from the Northern Seas that tells us that sailors in duty that are in great danger and love their ship are often rescued by those mythic creatures. Since men from the north were always in need of sailing and if the ship was misguided and/or poorly taken care of it would mean death for them, usually sailors expressed immense gratitude to their ships.
According to the legend, many sailors were rescued by them and this was the only time in which they could see the Klabautermann. Even though they were supposed to be good and mischief only people they didn't like, to see one was an omen that you were doomed. The traditional representation of a Klabautermann was a small sailor in yellow with a tobacco pipe and woolen sailor's hat, often carrying a caulking hammer.
There are a few movies/series in which we can hear of a Klabautermann, being One Piece probably the most famous one.
With this introduction, allow me to formulate 2 hypothesis
1) There is no creature in Tibia
We know that NPC's and even books are biased and even though they tell us important stories of Tibia, they tell what they believe or what they want you to know and not always this correspond with the truth.
I believe that when Marcus says that Ferumbras is the father of all Klabautermann he is meaning that once you saw Ferumbras you were doomed, since his powers were almost death sentences to anyone who approached him.
Not only that but both Marcus and Waverider are sailors and, as we saw before, sailors were the main source about these creatures, making a hard belief of it.
Even the book is in a place such as Nargor, a pirate's island so, again, full of sailors. Because of it, I really believe that this creature (at least for now) is only a myth that is passed by experient sailors to novice ones, perpetuating it. Not knowing any other place where we can check any information about them for me is pretty evidence that this is the answer.
If there is any chance to find any new information about it I'd agree with Mogh that Svargrond may be the best place to search for, since the original legend in real life is based on the Northern Sea. Since Svar is also based in the nordic culture, maybe there we will find any hint.
2) Captain Waverider is a Klabautermann
Okay,, before I even start, let me tell you that this is a long shot and I do not truly believe this is the answer. But there are certain similarities that may trigger a little doubt.
As we saw before, in the old legends in real life Klabautermann were described as a small sailor (checks) in yellow (also kinda check), with a tobacco pipe (ok, this doesn't check but I don't even remember seeing tobacco in Tibia at all) and a woolen sailor's cap (kinda check, I mean, there is a cap and I definitely can imagine its material being wool).
Not only the appearence is in agree but also the description of both the book and Waverider itselfs kinda matches. In the book we read that a Klabautermann remain unseen most of the time (thinking metaphorically, not literally, Waverider passes by as a common man through the eyes of the governors of Liberty Bay). We also read that you can keep a Klabautermann happy with lots of rum and Waverider sprite is the same of a pirate cutthroat, who drops bottles of rum as loot.
Captain Waverider also tells us that Klabautermann lives on ships (so does he) and if you befriend one he'll become an useful ally (so does he, helping you through The Shattered Isles Quest after you acquires his confidence and showing you the path to the secret Treasure Island.
As I said before, I personally don't think that this is the answer, but at least for now, as Klabautermann remains a mystery in Tibia, I think the mysteriando lets us to formulate some theories.
Hope my answer helped a little bit XOXO