How do you describe when a myth causes people to assume that something real is also a myth?

Is there a word or phrase for when something real is thought to be mythical because it resembles or is related to a known myth?

For example, the myth of unicorns makes many people think that narwhals are mythical creatures as well, when they're actually real. Another example is that the myth of Santa's reindeer make some people think that regular reindeer are mythical as well. Is there a term that would describe this?

To clarify: I'm not talking about things like creation vs. evolution or global warming's existence vs. lack thereof, where one side genuinely believes that A is a myth and B is a fact, and the other side genuinely believes that B is a myth and A is a fact. I'm talking about a situation where everyone knows that A is a myth, but A's similarity to B causes some people to erroneously conclude that B is a myth as well.

Solution 1:

In rhetoric, this is the 'weak analogy'. I quote:

"(1) A and B are similar.

(2) A has a certain characteristic.

Therefore: (3) B must have that characteristic too." See http://www.logicalfallacies.info/relevance/weak-analogy/

To apply this to your first example: Unicorns and narwhals both have horns and I've never seen one. Unicorns aren't real. Therefore narwhals aren't real.