One thing that annoys me when it comes to monster designs is when it comes to friendly monsters and hostile monsters, often times the friendly monster looks huggable and soft like a giant push toy, while the hostile monsters look more jagged and edgy than a punk rock concert. Now I get its the symbolism or metaphor thing blah blah blah, but I'm a guy that likes a little more realism. For example, crocodiles are scary to look at, but as long you don't display yourself as a threat, they're pretty chill. Meanwhile dolphins would kill and torture baby porpoises for fun, which leads to my second pet peeve.
Why are most monsters I see either very passive that they're basically sitting ducks or extremely hostile that it's like they are infected with rabies? Come on people, a normal dog wouldn't jump out of nowhere and constantly want to bite your balls off... most of the time.
This is probably why I like the dragons in How to Train Your Dragon.