Chiron

From space, it looks like an ordinary dwarf planet. No oceans, no atmosphere, no nothing. But all the gasses are under the surface.

A giant cave system worms it's way through the planet. The se caverns hold oxygen, protect from radiation and have oceans at the lower levels. Steam clouds drift upwards through the dougouts, eventually dripping back down as a strange form of rain.

The vegetation consists of slime mold like detritevores. But the planet is most well known for its fauna. 10 meter long arthropods, slimy skinned buggers and cannibalistic freak shows clutter together into a steamy pile of "i'm not colonizing that".