On mobile, the best place to look for otters is that large river that runs near the redwoods. Otters also don’t require whole fish like the Dododex says, just regular fish Meat will do fine. I do recommend you pick them up and bring them to a safe location though. My otter, Honeybuns, only had a health stat of 48 when I first got her.
I recently had tamed a low level otter in Ark Mobile. When ot was tamed it had the weight of 38kg. When I took the otter to the cave for some reason it didn't allow me to place artificts to the otter's inventory