Krynn are giant ocean beasts, basically a head with tentacles. They grow to immense sizes but mostly stick to the deepest part of the ocean where they can hunt freely.
The most notable encounter with a Krynn is known as 'the flight of the Borealis'
Where a single giant Krynn found the Spire farms and began feasting on the Spires themselves. Pilot Borealis overtook his captains ship, positioned it near the farm and using only his pilotable Worker Drone killed the Krynn.