The Dorudon is a prehistoric whale seen in the beginning of the episode "Ugh."
The Prehistoric Purple Fish, like the name says, is mainly purple in color. Its body highly resembles that of a whale. It has large light blue stegosaurus-like scales on its back and very short fins. It also has two small eyes with thick eyebrows. A few razor-sharp light yellow teeth can be seen jutting out of the lower jaw of its wide mouth. The top of its head consists of a blowhole and at least three hair-like growths. Its skin is detailed with scales.
The Dorudon is first seen in the very beginning of the episode "Ugh." It makes a noise which sounds similar to SpongeBob's alarm clock. It may act as SpongeGar's alarm clock every morning.
The Prehistoric Purple Fish appears to be the early ancestor of a whale.
The Dorudon's purpose is similar to SpongeBob's alarm clock, to wake up SpongeBob and SpongeGar.