Even those three examples, you can copyright the character concept if you can prove that you indeed originated that concept. In fact, with the Marvel characters you mentioned there are multiple copyrights, covering graphic elements of the characters and therefore products derived from those characters. The copyright documents often read like "graphic black spider element with elongated legs for use on the chest of a character's costume" and "Red and blue web pattern with central points on the center of mask and center of chest." Otherwise there is a pretty good chance of someone coming forth with previous art to invalidate your copyright claim. Also, you can copyright a character name, which is the route most content makers take.

Copyright filings cost a non-trivial amount of money, and copyright challenges are practically free. It is highly unlikely that a lawyer in his right mind would put together a copyright claim for "Blue Cartoon Shark with legs" or the name "Left Shark".

EDIT: the issue of Marvel and copyrights is something kind of near and dear to my heart from my days playing City of Heroes and following the attempted Marvel lawsuit against the game in its early days.