The Giant Axolotl lives near bodies of water in both the fertile region and bio-luminescent region. It is passive and will flee if attacked. Most creatures, including large predators, do not attack the axolotl.
Stats
The giant axolotl would have a large amount of health, stamina, and good weight. It also has high torpor.
Utility
Giant Axolotls emit a gas that makes creatures within a large distance of it immune to radiation, this includes survivors. This would allow you to take basilisks and tek creatures into areas with radiation as long as they are within range. This also lasts for a short time after leaving the radius. While giant axolotls are mate boosted the radius of this is doubled. Side note: build base in radiated areas.
It can also become slippery and slide long distances making it a good travel mount. They also reduce the weight of black pearls, element ore, and red gems while they are in its inventory.
Giant Axolotls can also gain the same buff that spinos get when they enter water. They also regenerate health much faster while the buff is active.
Combat
Giant Axolotls have a large amount of health which could make good tanks. You can also use its slide to leave behind a trail of slime that slow creatures caught in it, this does not include other axolotls.
Taming
There are two ways to tame a giant axolotl, passive or knockout.
Passive:
In order to passive tame a giant axolotl you will need to feed it filled fish baskets. Once it has reached about 50% the axolotl will begin to follow you and not just want fish baskets, but care as well much like babies.
Knockout
You can knock out a giant axolotl like any other regular creature, but it has a high amount of torpor. Once it begins to run most creatures within a large radius of it will begin to attack you as if you attacked one of them. This will end once the axolotl is knocked out.
Misc
They’re adorable which is just a good bonus.
Also I know they're not extinct, however they are on the verge of it.