The Ancient Race is a bestiary of massive and legendary creatures that predate the appearance of the mortal races such as man, dwarf, and orc. Each creature is invariably massive in scale, with the smallest easily being as large as 10 grown men and no real upper limit to their size. The mythical Leviathan, Behemoth, and the individual giants of lore would count as members of the Ancient Race.
Each member of the Ancient Race was created by the hand of one of the divinities. Given that the powers that created my setting were later betrayed, imprisoned and converted into demons, most specimens of the Ancient Race are terrifying, monsterous, or horrific. And most of them were slain in the conflict that put the elder gods in bondange and the usurper gods in power. A few yet remain, hidden and lurking out of notice.
Members of the Ancient Race tend to have strong innate magic, a high resistant to mortal cast magics, and unlike the mortal races do not require the same level and sort of substanance. These creatures are natural thaumavores and can subsist on ambient essence, though they have no aversion to actually hunting, killing, and eating as the mood strikes them. They are also sterile, and completely incapable of reproducing. They are also generally of high natural intelligence, but this intellect is often of a very alien design if compared to a mortal mind.
Additional Ideas (2)
While certainly unique to a given setting, the gigantic uber-sea monster certainly is not. The specific abilites of the SOL are largely irrelevant as it's primary purpose is to devour ships in single bites, and on occasion be mistaken for an island.
Some variations on a theme:
Giant Whale - Much like Moby Dick, this leviathan is a whale of superlative size and nasty disposition. Most likely a foe to be encountered in colder northern waters, it can be a regular foe to whalers, cold water merchants, and the force along with the ice, that causes the edge of the map to end in an open blank, scrawled with Here be Monsters.
Kraken - There is something profoundly disturbing about the boneless strength and alien appearance of a tentacle. The Kraken is a perennial favorite in mariners tales, from Jason and the Argonauts to 20,000 Leagues Under the Sea to the newest PotC: At World's End. This Kraken lives in deeper and warmer waters than the leviathan whale.
The Whirlpool Beast - Never seen, this beast lurks beneath the surface and when it sees something it would like to try munching on (say a ship) it opens it's gullet and starts sucking in water until it creates a whirlpool that ends in it's stomach. A creature to be feared, but really not ever seen.
Standard Scaly Serpent - Like the Loch Ness Monster on steroids, the big Triple-S has a long neck, flippers, and a penchant for eating sailors. A distant relative to dragons, this big guy is a warm water paddler and is likely to be the apex predator for an island chain, or achipelago.
Perhaps the best example of the subterranean Ancient appeared in the movie Tremors, where a small desert town was terrorized by 4 burrowing worms the size of school buses. Given the difficulty of tunneling and burrowing, most Ancients from below will follow a general worm-like profile; a blunt head, barrel shaped body, and reliance on scent and vibration over sight. Unlike the ocean, the domain of the soil is not nearly as crowded and there is more room for less predatory aspects of the Ancient Race.
The Giant Worm - akin to an earthworm, but several thousand times larger, the giant worm burrows about, conveniently digging out passages that are easily converted into dungeons by villains!
Big Hairy Spider - A god among the crawling hordes, this multi-legged horror has a tunnel miles deep and can grab not men, but animals as large as elephants and drag them deep into the cold embrace of the earth. as a prototype spider, it can have some wild variations, more than 8 legs, and so on. If inspiration is lacking, consider Shelob and Ungoliant from the Lord of the Rings and Silmarillion.