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.
They can destroy wooden structures. I was in the east underwatercave for deep SEA loot and Had a 1x1 wooden House with a bed. I had No weapon and so a lvl 200 dungbeatle Killed me nearly, Destroyer my bed and my foundation not what i expected.
On Ragnorak you can find them in the volcano biome beside the highlands . Very easy tame just use a couple of spoiled meats or dung. They have to be on wander to produce fertilizer, i keep mine in my greenhouse.
In Valguero I saw some of them on the bottom of a dragon nest along with Titanoboa's too and I believe Mantis too (don't remember too well) anyways I only found out because sadly ice dragons are gigantic jerks and froze me at the same time that my pteranodon ran out of stamina as I started to descend down to the lava I spotted the dung beetles before dying and losing my stuff. So if you have strong tames head to the snow zone and it should be around the edge of the top right side of your map once you see the ice dragons flying around there nest shouldn't be too far fly down and it should be there.
I find it best to go to the valcano on rag me and my tribe mate we’re able to tame about ten dung beetles there. I recommend bringing a Phiomia And some stimberries for quick poop. It can get a bit dangerous so also bring something with strength personally I just brought a wyvern and it worked fine.