For full Otter taming details, get the taming calculator app.
To tame an otter, kill some fish, approach the otter, and then use the action button to feed the fish to it. Level of fish does not impact taming. The Sabertooth Salmon is by far the most effective fish, followed by the Piranha and the Coelacanth. Please ignore timing and effectiveness as calculations are not yet complete.
Their a rare/ uncommon spawn. They are mainly found down south in max of a pair of two. But their usually found alone. They are a passive non-agressive tame that wont flee. (They wear hats too). Have fun!
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.