This model's a concept for the Fossil / Archaeology Mod by Flammarilva. You can view his thread here: [link]
Joining the Nautilus as a living fossil is the Coelacanth! As such, it can be found as DNA like the Nautilus can. To revive it, simply place the Coelacanth DNA in the Cultivate to receive the living specimen (Beware of Monster Outbreak!). Then with it in your inventory, place it in any body of water. Otherwise, wild variants can be found rarely in Ocean Biomes at deep depths.
Killing Coelacanths yield Raw Fish as the prize, an alternative to fishing. In addition, Plesiosaurs, Ichthyosaurs, and Mosasaurs will attack Coelacanths for sustenance. Coelacanths can be collected like Nautilus by right-clicking. One could attempt to eat the Coelacanth in this state, but would get a nasty case of food poisoning (similar to Rotten Flesh).
Coelacanth © Me
Minecraft © Mojang