A fish. Famous for once being thought to have gone extinct around 66 million years ago, but were rediscovered in 1938 off the coast of South Africa. Ovoviviparous, only two species survive, both critically endangered. They are easy to spot because of their strange appearance; however not all primitive or strange-looking fishes should be tagged as coelacanth. They are lobe-finned fish (from where tetrapods come), as opposed to the most common ray-finned fishes.
They are easily distinguished by their large rhomboidal anal fin, and their characteristic second dorsal fin which is lobed; ray-finned fishes either don't have a second dorsal fin, or it's not lobed. Actually, for coelacanths, all their fins are lobed except the first dorsal fin. Examples of coelacanths are post #470478 and post #974792 (at the top right, just below the sun).
(Its Wikipedia article)