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!
I have tamed 6 of them so far, all found along the Redwoods beach and in the river. They are very small and look like small bearers underwater but are easy to spot on the beach. I've only ever encountered one at a time, and never in the same place. They are fairly easy to tame, just pike fish (I recommend using a high level pike- work smarter, not harder) and walk up to the otter with the fish. It will prompt you to feed the fish to the otter, he will make an animation, and the taming bar should show up at the bottom. Don't worry about losing taming effectiveness as you go around gathering more fish, I don't think I've noticed a problem yet. On an Official PVE server
Until they start spawning regularly, or if your server hasn't done a dino wipe, just go on a killing rampage, kill everything near or in the river until one spawns. If the otter that spawns is too low lvl for your liking, kill it so you don't keep checking to see if it is a new one.
They spawn quite alot in the beaver ponds near castle on official. Two people, one killing salmon and one feeding otter makes this a very easy tame. The beavers also protect you from carnosine and other nasty predators. Hope this helps.