I have only seen these in caves so far but they are useful for fertiliser when you are growing crops and Plants Species X etc.

The dung beetles are often found in caves and around swamps

U can find loads of them in the center's lava island cave

On the island, I think the easiest place to find them is in the lower south cave. Go in with a longneck or a shotgun, turn left at the first intersection, kill the boas and spiders I. The first chamber, near the end of it there’s usually 1-3 beetles. Tame, and carry back to base w/ Petra. Easy fertilizer.