Adventuring in volcanic environments is dangerous in and of itself. It gets even harder when the lava comes looking for you.
Magma oozes are blatantly obvious encounters for any volcanic/magma/Plane of Fire and/or Earth-related encounters. Here are a few less likely places where they might be found…
A giant in life, the Pharaoh of Fire is more powerful than most mummies, and not just because of his size. The secret to his many varied special abilities is a set of enchantments placed on his organs, now preserved in canopic jars. Six of these jars exist: four containing his stomach, lungs, liver, and intestines, and two containing dormant poisonous magma oozes to roast and devour the unwary.
The expansionist dwarves of Tellus Hok do not regard non-dwarves as people. In peacetime this means refusing to learn other languages (especially Elvish and Gnomish tongues) and relying on dwarves from other subraces (“nickelbloods”) to serve as interpreters and traders. In time of war (far more common these days), this means abandoning the standard conventions of war in favor of extermination. The high dwarves’ most infamous tactic is catapulting water-cooled crystalline magma oozes into enemy strongholds.
Delving deep below the earth, adventurers discover the bodies of kobold and human slaves staked alive to the tunnel floor, forming a crude trail. The trail leads to a lost city. After the cataclysm of the Remaking, which sank a continent, raised a mountain range, and changed coastlines all over the world, Tashuar was one of many cities whose populace found itself suddenly miles underground once the dust—and their metropolis—had settled. Rather than flee for the surface or degenerate into morlocks (as so many others would go on to do), the Tashuar stayed put and adapted thanks to natural wonders—six pillars of lava that offered heat and light for crops in their new cavern home. But in the last fifty years, five of the pillars have one by one grown cold and dark. Now the Tashuar survive by attracting magma oozes to their cities, luring them with the trail of slaves into giant magical vats that leech the oozes for the heat that keeps Tashuar’s hope alive.
—Pathfinder Bestiary 2 184
Huh. In my head this was a short entry.
Let’s not forget the magma mephit! We covered him all the way back here.