To tame an Otter, simply interact with it (default "E" on PC) while dragging any kind of fish. Sabertooh Salmon is the best, Coel is the worst. Fish size matters more than type, so Megapiranhas are often worse than large Coels. When dragging large fish, chances are high that it will obstruct the vision while standing still; Just walk/swimm to the Otter and feed it before coming to a stop in order to keep the fish carcass behind you.
Quite common on Aberration, Otters spawn at large numbers from where the Fertile River delta splits in two (not going to the Fertile Lake) until its fall to the Luminous Marshlands. It's also worth note that the northern section has neither large predators nor Megapiranhas and Mantas on most of its lenght.
No other map has such a noteworthy spawn, but they can be found on most Redwood Forest lakes except for The Center, where it is unadvised to look for them.
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
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.