Pardon me as I wax metanerdy.
I imagine that most folks that read this have played D&D, and even if someone happens to be reading it that hasn't, you can easily imagine that its a game about killing orcs and dragons.
So anyway, about monster design in D&D. As I said, everyone assumes dragons and orcs and stuff. Check. Good solid monsters, close to "realism" - assuming that by realism I mean "Traditional Tolkien style fantasy". But the medium for Lord of the Rings was film*, while obviously D&D is an interactive game.
Let me pause here. I'm writing this because of course my brain churns furiously, lately, about D&D related stuff, and I saw this post by another blogger, about monster books, and about the difference in monsters that are "fun", and monsters that are designed mechanically. He's referring to a difference in approach. "Wouldn't it be cool if there was a monster that sucked people's faces off and had like four arms with lobster claws on them?!?" as opposed, as he references, to: "We need an aquatic abberation monster that fits into Challenge Rating 2."
I recently also read, and posted about this stupid monsters article. It rails against, I think, both types of monsters. Those created simply as filler, because they needed 80 monsters to slap between these covers so that they could sell a book for $19.95, which gives us such gems as the Owlephant and Duckbunny. Then there are monsters that were created with the Gygaxian theory that you can never make your player's lives too hard. Monsters that eat your armor. Monsters that pretend to be treasure. Monsters that pretend to be clothes. Monsters that pretend to be swords. Monsters that are the floor. Monsters that attack you from the ceiling. Monsters that pretend to be a cloak. Monsters that live in books and attack you when you read them. Etc etc etc ad nauseum. Jeff, in his previously mentioned blog post, talks about the fun of a monster that steals the map from an adventuring party. Yet another monster that exists solely to screw with the players (oh, and their characters).
And as I sit and ponder on these things, it occurs to me that sure, crazy ass monsters that eat your armor and steal your map are fun - in a way. It challenges the players as well as the characters, in a different way that encounters with non armor or map destroying monsters do not. I guess its all about the group, and the mindset. I mean, the rustmonster is easily a part of why we have satire like the card game Munchkin. I remember rustmonsters and mimics and things, and how much spice they added to D&D. Yet, as I've seen others discuss, I'm not sure whether I'd be hugely pissed, or able to just laugh it off and tell stories about it later - if the plate armor I'd just dropped all of my life savings on, got eaten by a beastie the next time I was out at the local dungeon. I think previously, I'd have laughed it off - not being of the questioning mindset, and that the DM/the rules were always right, and were just how it was. You don't get pissed, dude, its D&D! The rust monster ate your armor?! That's AWESOME! Let me tell you about this one time that my cleric found this succubus who was fighting a mimic......... Now I remember how fun stuff like level draining was too :P
So anyway, yet again, I run out of steam and wind down.
I own the 2nd edition AD&D book Monstrous Compendium Annual volume I. I love flipping through it, checking out all of the completely ridiculous monsters in it. I don't have it here, or I'd be more specific, but you've got (from memory)
A monster that pretends to be a house
An elephant-humanoid with two trunks and a bad attitude
Cats with antlers
A monster that pretends to be a sword (previously mentioned)
A man-purse that sucks your blood like a mosquito
A monster with a scorpions tail that pretends to be a coin
A skeleton monster
A skeleton monster on fire
Two skeletal arms
A mushroom monster
* Sorry, I don't know why I half-ass baited you, there.