On Valguero, these can be found within the Wyvern Trench in the northeast of the map. Go to the bottom of the trench's eastmost side to see a river of lava flowing into a cave. You'll find dung beetles in there, along with arthropleura, pulmonoscorpius, mantids, and araneomorphus. Visibility in the cave is low, so I recommend using first person while on your mount inside and maybe turning up the gamma. It is extremely easy to kill one of these while fighting other creatures in the cave, but if you spot one before you get to fighting, you can try to avoid it until you're done. I highly recommend getting a wyvern to help you do this beforehand, due to the dangerous nature of the area. They will help you avoid wyverns on the way in and out (which are territorial and may pursue you, and you can either defeat them or outrun them) and they will also help you in the cave. As long as you are mounted on a wyvern, arthropleura, pulmonoscorpius, mantids, and araneomorphus will not attack you, until you start attacking them. The minute you dismount, they will come for you, so you can use this to your advantage to defeat them and/or lead them away from the dung beetle you want. You can hold a tamed dung beetle while flying the wyvern. :) Good luck and happy hunting.

For those people playing mobile,

These aren’t only in dungeons, they are also found in the south cave AND lava cave! Trust me, I’ve seen them in there. The ones in caves usually want spoiled meat, but I’ve done passive tames and if it says, for example, (on a diplo) “feed azulberry to tame” you don’t have to. You can give it kibble instead. So I guess you can give dung beetles poo if they want spoiled meat. Raw meat also works to tame them. Oh, and have yourselves a good day with beetles! (Sorry it was long.)

~ screamingrangler