These are on extinction. To find them go to the northern entry of the sanctuary and keep going forward till you get to a pavilion that has turned into a swamp. Beware sarcos and kapros. Also, don’t bug the beezlebufos. Like so people can see.
Make a pen in deep, not too deep water, make sure the pen is in the water and not near land or going up to land. Tame a lot of piranha and I mean a lot. It'll be difficult since they come in groups in the wild and can break fish baskets with one to two hits, so becareful. Keep an eye on health and oxygen. But once in the basket they can spoil really quickly so hurry and release in the pen you built earlier. Keep collecting until you think there's a lot to do the damage. Now go into your pen and set all of the piranha on aggressive. Boom piranha pit. Now you can drop players or animals in.
These things are dummies. Just set your otter to neutral and park it by the water. Step into the water and wait for a piranha to bite you. When it bites get on land as the otter eats it. It works every time. LOL
You can use these critters for a pretty annoying yet op defense for your base, if you live next to a river or more less common living spot a small island you can surround yourself with wild or tamed ones. Especially the higher level ones. Once you have them surrounding your island or in your river. You have a pretty good defense for the river side of your base. Or all around your base!