Pac-Shark (or simply Sharks) are aquatic enemies that first appeared in Pac-Man World 2. They mainly attack by biting in all of their appearances.
They are basically round-bodied sharks with large sharp teeth and beady eyes.
As Ghosts, Pac-Sharks look pretty much the same except they emit a ghostly glow, have glowing eyes and their fins looks somewhat shredded and pointy.
As infants, they are smaller in size and have larger eyes, but still have round bodies.
Pac-Man World 2
Pac-Sharks only appear in the levels "Scuba Duba" and "Shark Attack". One will try to eat Pac-Man but can be killed by spinning into it.
"The Shark is a huge predator of the deep ocean. Feared by all creatures in the water, the Shark feeds upon the fear of its prey to attack with stealth and vigor."
Pac-Man and the Ghostly Adventures
In Pac-Man and the Ghostly Adventures, Pac-Sharks appeared in the episode Paclantis as Ghost Pac-Sharks. Throughout the episode they tried to eat Pac-Man and his friends, but they were luckily evaded. Near the end of the episode, one Ghost Pac-Shark ate the last Youth Berry, which turned it back into a living Pac-Shark and made it younger.