Megafauna

Often overlooked for Medieval Fantasy campaign settings is the placement of megafauna.  It's an element that's easy to integrate which has both a familiar yet alien feel.  Unlike more tranditional monsters from roleplaying game manuals, which require more consideration to place and explain, megafauna can be woven into regular biomes with only slightly more consideration than typical animal life.  And, of course, they are inherently awesome.