Not even sure if this is the right sub, if it isn’t I’m sorry, I’ll delete the question. Anyway, I took an Intro to Mythology and this question was posed, but a specific answer was never given.
I think it’s really interesting because a lot of cultures have very similar stories of creatures like werewolves/shapeshifters, or vampires, kraken, etc. And some of these cultures has these similar stories long before they were in context. For example, shapeshifter/werewolf legends can be traced back pretty far—quite a few mentions of it in Ancient Greece, but there’s also mentions of it in Native American tribes from not long after, and there’s no way these two cultures ever came into contact.
And there’s tons more examples of these weird similarities, so I was wondering what the cause is?
There are two basic ways to answer your question: there are some deeply-rooted Paleolithic beliefs or these are traditions that various cultures created in parallel. Let's break down the three examples you provide into general categories of shape shifter, walking dead, and sea monsters.
On shape shifting: many, but not all, cultures have the idea that people can turn into animals and animals can turn into people. Common subjects for these beliefs include birds, bears, wolves, seals, wild cats, and deer. Sometimes creatures appear as hybrids of animals and humans (mermaids for example). I don't think we need a paleolithic root to explain these various beliefs, and it is not surprising that cultures that believe these transformations are possible settle on likely animals commonly in their midst. It would be difficult to prove a paleolithic root since we would need to probe deep into a realm without documents. I don't feel the need to imagine that foundation culture to explain the diverse traditions. At the same time, I have no evidence to rule it out, but since I can't prove it exists, imagining it can be little more than a fun parlor game.
The same is really true of various traditions involving the walking dead. Those who find mobility after death are usually imagined to be either spiritual, ethereal forms or walking corpses. These ideas are international, and frankly, I don't think we can find them to be too surprising since death is a startling part of out reality as living organisms. Internationally, there is a lot of folklore about death, and some sort of survival of death is embedded in traditional beliefs. The idea that some would not only walk but also crave human flesh seems to be a fairly predictable thing to imagine, so it is not surprising that many cultures include this as part of their belief systems. Again - Paleolithic root? Perhaps but the same as above.
And fearing - and therefore imagining - large monsters in the expanse of the open sea is again fairly predictable. The human imagination abhors a vacuum and the ocean depths are a perfect place for the imagination to wonder. The ocean is huge; it stands to reason that big creatures live there. The ocean is dangerous; it stands to reason that those creatures would be dangerous as well.
People often attempt to "connect the dots" in diverse bits of folklore separated by vast expanses of geography. Usually, these similarities are coincidental, and when examined closely, they aren't really that similar. A recent work - D’Huy, Julien and Berezkin, Yuri E. “How did the First Humans Perceive the Starry Night? – On the Pleiades.” The Retrospective Methods Network Newsletter 12-13 (2016-2017): 100-22 - suggests a paleolithic tradition as the basis of international stories associated with various constellations. It is an intriguing approach and a fascinating idea, but in general - as indicated, paleolithic foundations for folklore are either impossible to imagine or not needed to explain similarities.