Like, what sorts of traits can they have that can be easily depicted in art?
Which is useful if you want their first actual appearance to be on a fresco in the palace, to give a country bumpkin something to look at and the city slickers a way to sneer at him. (Not that they need much encouragement!) Gives them a chance to info-dump a lot of information in the guise of jeering at the very notion that it could be accurate.
Make it look monstrous is obvious. Showing the heroes depicted with glowing swords might help. But if, for instance, it magically inspires fear. . . well, you could show people cowering. . . .
Perhaps the tale is already known, and the character can interpret in light of the iconography. Which requires some slithering in of information. But does fit the habit of most cultures.
To be sure, some or all of the monsters on the wall may not appear in the course of the story. I could just have characters swearing at the difficulty, with all the history they have, of being stuck with something new.