The Great White shark that lives in the chemical factory, and swims in a tank. Madeline had just been rescued, and it all ended happily. Well, almost...
Death of Shark
He obviously has a reference to Jaws, as he killed Marge in the sequel to Jaws. Anyway, he possibly dissolves as the chemicals dispersed.
The shark also has a reference to the spinosaur from the third Jurassic Park.