Little species I made up.
Lake grabbers, (name pending) Sit at the bottom of lakes, like rocks. And wait for there prey to get closer. Deep in the river they catch fish and chomp down on there heads.
There fatty neck has lots of extra skin, This lets there head spin 180 degrees.

In shallower waters, they snatch out at small animals and drown them underwater.

They have been known to snatch children or small humanoids if they get to close, mistaking them for other creatures.

Best way to avoid them, is remain motionless. They have poor eyesight and think they have snatched up Debris.