My favorite is land sharks. They’re my home brew.

They’re basically a modified version of a shark from the MM. They were created by a powerful but reclusive wizard ages ago, and set loose in the valley he called his home. A few have escaped, but they’re mostly confined to that valley still

They can swim through any soil, but can’t get through solid rock. They hunt by sensing even the lightest tremors on the ground, then breaching from beneath their prey. They are, obviously, damn near impossible to actually fight. They don’t stay above ground long, and don’t disturb the ground as they swim through it. They don’t even swim at the surface when hunting, so you don’t get the warning of a fin cutting through it.

When I had it written down, it was a template that could be applied to any shark, though my favorite one to terrify players was a megalodon. The first encounter anyone had with them, their pack mule got swallowed, and it was just this spray of dirt as it breached, grabbed the mule and was back below in seconds. It was not the only shark in the area, so it got worse as they scrambled to try and figure out what was going on and how to deal with it. They ended up losing two horses before they figured out what was attacking; the Wizard made a good knowledge check to recall having heard about them, and they found a way to get into trees with their remaining mounts being driven into the woods that the sharks couldn’t breach through.

I don’t break them out often, but it is so fun when I get to use them with someone new :)