Before going into superheroes, Marvel published a lot of monster comics. This carried over into their superhero titles. Indeed, the first issue of their first superhero comic featured a giant beast menacing the heroes. Moreover, Marvel actually made monsters into heroes, or at least anti-heroes, like the Thing and Hulk. It was all quite bracing compared to the whitebread comics DC had been doing for so long.
Anyway, unsurprisingly a lot of their villains were monstrous too.