If you have an Argy, fly over the otter and pick it up with the argy's beak ('C' on pc). Land the argy and let the otter hang out in its beak while you bring fish over. I found I needed to drop the fish and pick it back up to target the otter. After having done this by following around a scampering otter with a dead fish pinwheeling wildly in my hands for 10 minutes the first time, I will never ever attempt it again without my argy.
(I’m on mobile) I just built a box without a lid (1 thatch foundation, 3 thatch walls, 1 thatch door and 1 thatch doorframe) by a lake so I could fish for fresh food to tame then I hopped on my pteranodon and I followed the river around the red woods (without the swamp) to find one of these lil guys and pick him up then took him to the box I made and put him in it. Turns out the for some reason the otter wanted COOKED fish… luckily I had some already so I just fed him that and some soothing balm and he tamed in about a minute. DO NOT READ THIS TIP THINKING ALL OTTERS EAT COOKED FISH IN MOBILE. Up if you found funny or helpful
These little guys are so much easier to tame in mobile, but harder to find. You still need to kill a megipirahna, but you don’t need to grab its corpse to feed the otter. After harvesting its meat you can easily walk up to it and feed it until it’s tamed. It doesn’t run away in mobile, and I’m sure you can use any fish meat.