1: THE SEA BEAR:
A Sea Bear is a large piranha-like fish with claw-tipped fins and the head of a grizzly bear. Squidward did not believe in the sea bear's existence until he was attacked by one in The Camping Episode where it is featured as the main antagonist.
The sea bear is quite disturbing for a kids show.
It is an exceptionally violent animal, the sea bear took an immediate dislike to Squidward and attacked him repeteadly throughout the episode.
The sea bear then violently mauls him and repeats this five times after for differing reasons: running, limping, crawling, simple dislike for the octopus and for drawing an oval rather than a circle in the dirt when Squidward takes Spongebob's suggestion. Conceding, Squidward jumps in Spongebob and Patrick's circle to escape the bear. The animal prepares for another attack when it notices the dirt circle.
Angered at not being able to attack anymore, bear leaves, but not before giving a menacing point towards Squidward..
2: PUFFY FLUFFY:
SpongeBob bought him so that Gary wouldn't be lonely while he was at work, but he turned out to be a monster and attacked Gary several times (while SpongeBob was oblivious to the entire situtation). SpongeBob never notices and blamed Gary for the desruction of his house, which was really caused by Puffy Fluffy. Puffy Fluffy sheds his skin during the night and turns into his fully-grown monstrous form, resulting in Gary battling him throughout the house the entire night. When SpongeBob starts to scold Gary, ignoring the fact that Puffy Fluffy is now trying to devour the snail with his tongues, the monster decides to eat him instead. Gary saves his owner's life by tangling up Puffy Fluffy's extra mouths with his own leash, preventing him from eating and sending the monster running out of the house in a sobbing panic. Even so, SpongeBob still blamed Gary for chasing Puffy Fluffy away, even though Gary ended up saving his life and the house.
3: Patrick Star:
He is the best friend of the titular character Spongebob and he is a slow-witted Starfish. Despite being a "protagonist", Patrick's stupidity can cause him to sometimes indulge in unethical behavior. He has frequently served in a villainous or antagonistic capacity, though often unintentionally.
Patrick often proves to be a bad influence on Spongebob and is also sometimes the reason he gets into trouble. Patrick has become especially antagonistic in post-movie seasons when his antagonistic nature appears, at which point his stupidity is flanderized along with other characters' personality traits. Worse, his obnoxiousness and idiocy eventually ascended into disastrous level, ranging from annoying people to causing destruction.
4: UGENE H. KRABS (Mr Krabs):
Since the beginning of the series, Mr. Krabs's overall characterization has become more and more negative. Sometimes, he becomes the villain in some episodes of the show, with Mr. Krabs being just as bad, if not worse, than Plankton himself. After the movie version of the series released (specifically 4th season), Mr. Krabs' role as a father figure to SpongeBob is pretty much completely destroyed, and he is pretty much considered a full-blown Protagonist Villain now. He has a recurring tendency to exploit his employees and even exhibits sadism towards Plankton upon the latter's defeat. For a very good example of Mr. Krabs as a prominent antagonist, one should watch the episode Born Again Krabs (though he does show signs of redemption near the end) - another good episode to consider is Jellyfish Hunter and One Coarse Meal - which all show Mr. Krabs being at his worst.
The explanation of his antagonistic nature was his obsession with money eventually drove him insane, which flanderization of his personality in post-movie seasons clearly shows this.
A hideous, demonic version of Mr. Krabs that appeared for one second in an episode of the show and now is seen frequently in videos on the internet is Moar Krabs, reflects his monstrous nature and indifferent to suffering toward anyone and anything but money.
5: TATTLETALE STRANGLER:
Although you may probably argue he a pretty enjoyable character.
Keep in mind this is a kid show.
And they actually go as a far as a "murderer" out there, and there's a very dark line he uses when he thinks he's alone in Spongebob's house.
Saying "Now your gonna get it.. Tattletale" while holding the dimwitted Sponge by the throat