Does Anyone Smell ... Smoke? In a certain dungeon, every time someone starts lighting a fire or casting a fire spell a large humanoid bear in a hat appears and beats them down with a shovel.
PCs may be a little ... put out by this, but if they manage to slay said bear (should be hard) they discover that the entire dungeon, which appears to be made of the standard large unmortared stone blocks, is highly flammable as it burns down around them.
Ancient Enmity. PCs are hired to clear a forest of owlbears by powerful logging magnate who wants to harvest the timber. They meet the first owlbear and it is an adorable 4" tall critter that lives in a small hole under a tree. It and ~10,000 of its friends form into a huge burrowing owlbear swarm that either kicks the PCs butts or hires them to clean the logging magnates out of Town.
Barbearian Invasion This might be bear barbarians, regular barbarians mounted on bears,
ice weasel adventurers on polar bear based fighting platforms, or a horde of 10,000 burrowing owlbears who are feeling extra fighty now that their natural enemy the loggers have been put down.