*checks under bed* Just the laundry-drying rack under there now, as expected, nothing snuck under while I was browsing CFC!
That said, definitely Stegasaurer, I've been a fan of the stegasaurus since around 1st grade, and no monster wants to deal with a stegasaur.
Probably Specialist Chomp, naval warfare sounds about right for a mission under the bed. Plus hammerhead sharks are scary, to monsters as well.
Then probably Tiger. When I was growing up, the only actual monster I found under my bed happened to be our house tiger, also known as a cat. For some reason she liked to go under the bed, probably because it was a small space, and also provided a very well-sheltered, and cool-in-the-summer, place to nap. Being surrounded by a headboard, a footboard, and the set of storage drawers on the front, it was a little challenging for Tiger (and other monsters) to get under the bed, especially as a full-size tiger rather than a kitten, but that probably just made it more fun.
So every once in a while I'd be going to sleep and hear a meow or see a tail from under the bed - it was Tiger. Tiger was of course not at all scary to me, although she did her best to be scary to other cats. But with her proven record of success at keeping other monsters out from under the bed, she's be a solid part of the team.