These things will spawn in your water pen and kill your tames if they're on passive. Make sure you have Dino's to kill them or foundations all around your pen, they've killed so many of my Icthyosaurus' it's not even funny.
I'm around lvl 30 and ever since I started the game I can honestly say "I hate these creatures!" I've died by them more then anything else lost two parasaurs, a raptor, and who knows what else. Advise keep glancing under water to see if any are around, avoide rivers crossing on foot. I like rock hopping and dino swimming they can take more damage
When entering the water, go under and look around for a medium sized fish lunging for you. If there is, swim back to shore asap. The piranha should be near the edge of the water. Pumch it without getting too deep or close to it. You can easily gather fish meat this way.
If you’ve got a sheer enough drop off into deep water, these are easy and hilarious to kill. I was dehydrated in a cave one time, stepped into the water to see 20 of these guys turn to me, stepped out and they all popped out after me. They die the second they hit air.
A nightmare for lower levels & those that cross rivers frequently with land-based mounts, but can be easily dispatched of once you have a decently capable amphibious mount like Baryonyx or an Ichthyornis. However, if neither of those are available, a Parasaur will do to avoid them.