If you think about it, I mean really ponder it, the only monster you really need is the Flumph. It's so obvious to me now.
Seriously, there are just a ton of critters that I've never used, nor would I ever plan on using. Probably I'm just bitter because my players bypassed two gargantuan toads of chaos which burped up Slaadi in my last campaign.
I've decided that I'm tired of humanoids. They're just so overdone. Orcs, goblins, kobolds. Give me some bullywugs, or something at least a little more interesting.