We already got a better luvdisc, though. Relicanth and Kecleon would be neat. I could have added a bit more detail to the frontal armor, but here's my take on a Relicanth evolution. That last sprite looked like a Plesiosaur, which isn't even a fish anymore. O_o
Name: Relidira
Species: The Endurance Pokemon
Pokedex Entry: Relidira have tough armor that allow them to withstand the crushing pressures of the deepest ocean trenches. Though slow, they never stop swimming. Nobody knows how long they can live.
Name Origin: From
relic, meaning something old that has survived the passage of time, and
arthrodira, meaning grim joint. (The order of ancient fish that contained Dunkleosteus.) Also relating to
dire.
You can justify an evolution if you make them live in even deeper holes than the last, right? Sure, Dunkleosteus and the Coelacanth aren't really related, but neither are most axolotl likely to evolve into giant salamanders.
Base stats:
HP: 100 → 105
Atk: 90 → 100
Def: 130 → 140
SpA: 45 → 45
SpD: 65 → 90
Spe: 55 → 65