I don't mean "did some people believe", I mean more like did states actually make decisions based on these things?
Like taking different sea-routes because they thought sirens would hypnotize them or took a different trade-route because they believed there was actually a dragon eating people?
Or has it just always been used as some sort of "scary stories" to make sure people do what you want?
Saw an old map with the "here be dragons" and wondered if they actually believed that or if that part was just dangerous in general.
I found this answer by u/wedgeomatic in the FAQ that kind of answers your question, though it looks like there is a lot more to be said.
While you wait for fresh answers from qualified experts, you may be interested in a previous response on the topic.
Why are dragons a common association with the Middle Ages? Did medieval people really believe in their existence? with a response by /u/depanneur.