To write down the first problem, he only existed in this episode as a pet for SpongeBob to buy because he felt that Gary was "lonely" at home while working at the Krusty Krab when the snail was actually having the time of his life before being introduced to Puffy Fluffy.
What also makes him pointless is that SpongeBob already had a pet scallop (something which said episode he appeared in had completely overlooked), so it means that Gary was technically home with someone already.
His only defining character trait is nothing more than a vicious, savage beast who loves devouring any kind of animal besides his own species, including the nudibranch family of consuming sea slugs as seen when SpongeBob adopted him, as well as terrorizing Gary.
He seems to enjoy attempting to devour Gary when SpongeBob isn't around. He also somewhat frames him because of SpongeBob's obnoxious delusions and sheer idiocy.
Similar to Tom's Owner of the Gene Deitch era, he is purely toxic inside and outside of his appearance. While he might be cute at first, when SpongeBob (or anyone else) isn't around, he turns into a vicious monster who shouldn't be trusted at all, giving him some kind of a suspiciously split (or two-faced) demeanor ala Dr. Jekyll and Mr. Hyde, but NOT in a good way though.
Speaking of his monster form, when he was in latter form, he wrecked SpongeBob's entire house.
Also, it looks way too off-putting and nightmarish (like some minor characters in these seasons) even by SpongeBob SquarePants standards, and it looks more like a rejected The RealAdventures of Jonny Quest or He-Man and the Masters of the Universe character than a character from this beloved show.
Despite him being a boy, he has eyelashes and a blush, so he looks like a girl instead, which can make his "normal" design look a bit confusing. Some people could believe that he was supposed to be a transgender creature, due to him looking feminine.
He's also just a generic and disgusting rip-off of both Stripe from the Gremlins franchise and the Xenomorph from the Alien franchise while the Twonkies from The Adventures of Jimmy Neutron episode "Attack of The Twonkies" are similar to the Gremlins, they are well-executed parodies unlike Puffy Fluffy.
He does get his comeuppance for his actions after being whipped in the face by Gary, getting his mouth tied up, and then running away.
As said, his design when he's not in vicious mode, is at least kinda cute (if not a little ugly looking).
He did give SpongeBob his karma after he said "Bad Gary! BAD!"
As mentioned above, it is most likely that he and the episode he stars were meant to be hated due to how much cruelty Gary gets.
Fortunately, he did not appear again throughout the rest of the series, and probably will never appear again, because he was only episodic.
Outside of this episode, he also appeared in a late 2010 flash game called "Gary's Revenge".
A light-blue version of him reappears in the Season 12 episode "Gary and Spot".
His original form looks similar to a piranha.
The multiple tongues inside of his mouth are a reference to the Aliens and Tremors.
His species is never referred to by name in his debut episode; it is known for model sheets.
He is the only main episodic character from season 7 who does not appear on the cover of the Complete Seventh Season DVD of SpongeBob SquarePants.
In the Croatian dub, his name is "Čupavko", which means "hairy".
He, Fred, Scooter, Old Man Walker, Sandals, Elwood, and Jody Ann Chovy are currently the only incidentals with confirmed names.