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.
Once I went out to tame Raptors, or... try to shoot one from the safety of my raft... but then I saw a lil otter, my heart melted!!! Then I realized, the OTHER things that were in the area included: A baryonyx, a pack of 7 Raptors, A theri, and sum other aggressive friggin things, and the otter STILL LIVES THERE. Soooo many trikes and parasaurs died to the pack of FREAKING 10 RAPTORS! But the otter. Just lives. How.
Well... I am a otter god... I am here to message all those who may have "acidenantaly" Smooshed my children with a a horid creature. But I will find you. I will Murder you. I will have millions of my children eat you. We also have an alliance with the dodos and all shoulder mounts. We do not take murder lightly this is the first warning.
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!