Basilosaurus

Basilosaurus (a.k.a. Zeuglodon) is a genus of primitive whale from the Late Eocene. The typical species were around 18 meters/60 feet in length in life, and had serrated, triangular teeth with two roots, and had a second pair of small, possibly functional flippers. Together with other basilosaurid whales, Basilosaurus is frequently referred to as a transitional form between the four-legged, often semi-terrestrial primitive whales and "modern" whales, much to the dismay of creationists.

As a result, Basilosaurus actually comes up fairly frequently in debates with creationists. Notably, in debates between PZ Myers and religious dogmatists - Basilosaurus is one of his favorite examples of a transitional form.

The fossils are rather common, being found in Late Eocene strata of Egypt (in the Valley of the Whales), Pakistan, and Alabama and Mississippi. In 1845, one self-titled "Doctor" Albert Koch unveiled a 35 meter/114 foot long skeleton, titled "Hydrarchos," ("water ruler") built from the remains of 5 fossil whales, including some Basilosaurus. Hydrarchos "went extinct" by being one of the numerous casualties of the Great Chicago Fires of 1871.

Reptile or whale?
When Basilosaurus was first discovered, its discoverer Mr. Richard Harlan erroneously assumed Basilosaurus to be an extinct form of marine reptile (specifically with vertebrae resembling that of a plesiosaur, with a skull resembling that of a mosasaur). Richard Owen corrected Harlan on this, and being the arrogant, unscrupulous bastard he was, proceeded to take credit for the discovery of Basilosaurus by renaming it Zeuglodon, meaning "yoke tooth" in reference to the fact that Basilosaurus still had the traditional set of mammalian teeth, molars, canines, incisors and all. Of course, that's not how taxonomic nomenclature works, and much to Owen's dismay the name of the genus remains to this day Basilosaurus, even though Owen's preferred name is arguably more correct as a descriptor than Basilosaurus.

As a cryptid
Cryptozoologists feel that some lake monsters could be a relict population of Zeuglodons. Apparently a 30-million-year fossil gap isn't that big of a deal.