To be honest, the hungry fog feels more like a metagame exercise (Q: When is an undead fog not undead? A: When it’s a negative-energy infused ooze) and a publishing math problem (If: There are 22 undead in the Bestiary 3, and If: There are only six other oozes, Then: To fill out the roster, the hungry fog shall be labeled an Ooze) than it does an organic creation.
That said, it’s a nasty exercise for all that, with a host of resistances, immunities, the usual ooze traits, and a gaseous form that makes it hard to escape. It’s also a nightmare in any undead combo encounter; with its negative energy touch it can turn even low-level undead threats like ghouls and wights into devastating regenerating nightmares.
One final thought: As bad as they are on land, hungry fogs are especially lethal at sea. Though a hungry fog fears storms, it can easily lurk in the shelter of a cove or the hold of a ship run aground. And while most PCs of an appropriate level can stand up to the CR 6 monster, their crew is another story. By the time the PCs act, a hungry fog with surprise on its side will have already sapped the life out of the men they need to run their ship, stranding them to face the tender mercies of its regenerating draugr or lacedon allies.
The lighthouse at Lonely Rock was a crucial landmark for shipping. Then the galleon Hermione crashed on the reefs, releasing the hungry fog that had sheltered in its hold and devoured its crew. The fog made short work of the lighthouse keeper and her family, then found itself trapped by the ever-present wind. Unaware of any of this, the local mariners want the lighthouse beacon lit again, but having lost two candidates to whatever is lurking in the tower, they turn to adventurers for help.
A tiny principality caught between two warring nation-states, Elverford has been decimated by dueling armies. Such loss of life has generated at least three banks of hungry fogs. This in turn has caught the attention a flight of jyoti adventurers, who feel that the only way to contain the abominations is to put the oozes—and the hapless people of Elverford—to the spear.
Not all battlefields feature weapons of steel. On the eve of the Grand Duke of Vinland shifting the nation’s capital from Dernhold to Amberton, the Vinland Assembly held an acrimonious debate about the move into the wee hours of the night. The vituperations hurled back and forth attracted the notice of a hungry fog, which entered the sealed chamber and hastened the Assembly’s figurative demise into a literal one. After a month the hungry fog still lurks in the abandoned parliament building, but its torpor from consuming so many politicians has faded and it hungers once more.
—Pathfinder Bestiary 3 152
I should have mentioned this back in the black pudding entry, but oozes plus boats equals awesome. The tight confines rob PCs of their best defense against most oozes—mobility—and if the ooze shares the same vulnerabilities as the ship (to fire and acid, especially) the need to destroy the ooze versus the need to not destroy the boat is a great battlefield dilemma.
Or, to put it this way: Are you under 25 or over 35? If so, Google The Pirates of Dark Water and drown in the awesome.