It is tamed in the same way as the Hesperornis, where you have to carry a dead fish to it. Larger fish, and those caught by Ichthyornis, apparently work better. Piranha are better than coelacanths for taming, and salmon are better than piranha.
Just push this cute lil creature in front of a giga and it'll have an existential crisis. Realising the path of destruction it left in its wake and deteriorate in front of the otters blue sparkly marble eyes
They move pretty fast and you can loose them easily, so I suggest making a pen to put them in. Having a Ichthynoris is quite useful to catch salmon as you don't need to be in the water fighting them, the bird will snatch it right out of the water and bring it to you.
I've found my 5 otters in two spots. The first two where the beavers spawn near the waterfall area where there's that one cave (27,48.4) on the top of the waterfall the little water area below the beavers & then the other with the beavers above. Second spot (no cords, sorry) near the blue OB where there's that big area of water with that huge weird tree in it. Hope it helps!
Otters likes shalow lakes and watterfalls. Best way to tame otter is to kill all salmons around it, cause corpses has long time before it disappears and straight away feed them without any intervals cause they dont have cooldown on feeding time. After tame pick it up and you will get insulation from cold. Leveling otter is best to give all stats to oxygen as it increase insulation resistance. And you will forget biome struggle :) good luck finding them.
They may be adorable, but if you hurt them or their fuzz buddies, they will become angery, and can actually do pretty decent damage. I'm a level 100 player with around 250 HP and when unarmored, 7 can kill me in seconds. They also can do a number on your armor. Otherwise, they are cute and cuddly and friendly.
Bring an ichthyornis and some grapping hooks with you, find your otter then find a concentration of salmon nearby. Go grapple the otter and bring it to the salmon then tell your ichy to bring you bodies and force feed your leashed otter ;) also you can put the ichy back on your shoulder and bring your otter still leashed back to base on your flyer