If you want an idea of an Epic Monster, please look at the Super Mutant Behemoth as a model of what you could expect. As for random encounters, they could be anything from a random talking goblin to a group of camping bandits in the wilderness.
http://lh4.ggpht.com/_1i6mPvaLED4/SpqslHY9rBI/AAAAAAAAA3U/6bjFpOsS0kQ/ScreenShot243.jpg
http://www.thiel-a-vision.com/wp-content/uploads/2009/06/fallout05.jpg
http://images2.wikia.nocookie.net/__cb20090615000537/fallout/images/3/32/FO3_Super_Mutant_Behemoth.png
Let there be discussion!
EDIT: I edited the OP to correct a few things.