Somewhere between civilization and lost worlds lies the wilderness.
Lost worlds have the convenience of being just that—worlds unto themselves. Whether high plateaus, lost continents, or even the center of the planet itself, they are self-contained, discrete things.
But wilderness…that’s something else entirely. It’s not a place; it's just where civilization isn’t, and then it goes on getting wilder and wilder. And eventually, if you go far enough out, you find megafauna…beings just as out of time as dinosaurs, but more liberally sprinkled over the landscape. Apparently, some things just refuse to go extinct or hide out on the Isle of Dread.
At least that’s how I’d play the primitive whale basilosaurus and the monitor/iguana-like megalania. In a super-low-magic, Allan Quatermain-style campaign, these might be climax monsters: “Good lord! The legend is real!” But in campaigns where fireballs, wyverns, and T. rexes are commonplace, megafauna aren't exciting because of their stat blocks; they’re exciting because spotting one means your PCs are farther away from civilization than any man, woman, or grippli has ever been before. When a sperm whale capsizes your boat, you might at least be near a shipping lane. When a basilosaurus does the same thing, don’t look for a rescue.
Adventures are dispatched to rescue the commodore of the HMS Manticore, left stranded on an island after a mutiny. Investigation reveals his campsite but no commodore. To receive their full bounty, the adventurers must carve his remains out of the belly of the megalania that devoured him.
A sunken sea in the Lands Below hides pods of basilosauruses from the world. Regular clashes with charda whalers make these primitive whales leery of humanoids, and they are quick to smash small craft.
Peacock Jim, the head of the thieves’ guild, is notoriously agile and hard to kill, courtesy of years of acrobatics and combat instruction. But the dexterity-sapping venom of a megalania might be just the thing to slow him down…assuming one could be found and successfully milked.
—Pathfinder Bestiary 3 193
An alternate (and possibly more accurate but less official) name for the basilosaurus is zeuglodon. 3.5 fans can find an alternate take on the zeuglodon in Frostburn, a sourcebook I happen to like.
I had a whole tome on theories of wilderness left over from a Sacred Geographies class, but it was borrowed and never returned (along with a few other books) by a girl I went on, like, two dates with. If anyone knows a half-Ecuadorian girl named Michelle who likes college radio, feel free to liberate them for me.