Reaper Leviathan

The Reaper Leviathan is a large, hostile creature that does not yet spawn naturally, not to be confused with the Sea Emperor.

The Leviathan's model can be seen here.

Early animations for the Reaper Leviathan are available on Trello.

The Reaper seems to move around its prey for a few seconds before attacking. In the future, it will spawn around and defend the crashed ship. Currently the Reaper doesn't harm the cyclops, though it has enough strength to push it.

It can currently be spawned in experimental mode using the command SPAWN REAPERLEVIATHAN.

If fins and head protrusions are included, the Reaper Leviathan currently is about 2.5 times as high and about 2 times as long as the Cyclops.

Abilities
The Reaper Leviathan is able to instantly kill any players it attacks outside of their submarine. An early version of the Leviathan's player-kill animation can be seen here. The Reaper Leviathan is also able to grab the Seamoth and shake it around. Early Seamoth-attacking animations can be found here.

As of 4/3/15 the Reaper's animations are implemented. If you approach it with the Seamoth, it'll damage your Seamoth down to 50% health and let go. Next try it'll damage it to 5% health and let go. If you try again it'll eject you from the Seamoth. Once it lets go of the Seamoth it is too damaged to enter, however beware of attempting to fix the Seamoth as the Reaper will be able to eat you.