I think Ark needs more amphibians. And what better Amphibian, then Koolasuchus Hyperboreus? It's a large amphibian who's name translates as "Kool's Crocodile from the Artic". Also, I want to give a big shout out to DjayMasi and BangBooDoragon for helping design this amazing creature!
Behavior: The Koolasuchus will normally patrol the cold rivers and lakes of the arks. when it spots potential prey, it will blast them with a freezing brine and then will come ashore to scoop them up with it's jaws and then it will drag them down into the freezing water so it can feast on it's flesh. When it is low on health, it will freeze itself and will begin to heal itself, similar to that of the Snow Owl. But unlike the Snow Owl, this healing uses food instead of Stamina, similar to that of the Daeodon. But out of these three, I think the better healer is the Daeodon. But unlike the Daeodon, it is basically a aquatic medic. Those bumps on the sides of it's body can release a special mist which can be used to heal nearby creatures. It functions similarly to the crags from Natural Selection. The bulges on the sides of it's body are used to store great amounts of water and can us either to spit either a cold brine that can freeze foes, or a toxic goo that has a chance to knock them out. It has a very powerful bite and can carry any creature that has a drag weight of 150 or less. It can carry flyers while on land and aquatic creatures while in the water. In this state, it can only use it's bite to damage the target that it is holding. However it does have a powerful tail swipe with some good knock back and can do a belly flop that can cause massive amounts of damage and has a chance to stun targets with a drag weight of 150 or less.
Taming: In order to tame a Koolasuchus, you need to weaken it so that is forced to heal itself, then you need to passively feed it Meaty tributes. Here is a list of the recommended tributes from least effective to most effective:
Alpha Carnotaurus Arm
Alpha Megalodon Fin
Alpha Karkinos Claw
Alpha Tusoteuthis Eye
Alpha Reaper King Barb
Alpha Leedsichthys Blubber
Once tamed, you find that the Koolasuchus is a vary useful creature and it can be ridden on by using a very special late game saddle. This saddle not only reduces it's weight, similar to the Hyeanadon's meat pack, but it also acts like a mobile refrigerator. But be careful, it is a very gluttonous omnivore and will ravenously eat anything in it's inventory. Although, it normally prefers eating tributes over anything else. Fortunately, you can access it's inventory and will be able to access a selection wheel that will allow you to choose what the Koolasuchus can, and can not consume. Plus, it's powerful jaws make it a excellent companion to help you with collecting valuable tributes. And something neat about the Koolasuchus is that it will passively produce Organic Polymer, similar to how the Basilosaurus will passively produce oil.