It was long rumored that Gary Gygax once purchased a plastic bag of those little plastic dinosaurs (of the type you purchase in hobby and toy stores) and discovered a few odd critters in the bag. He promptly made them into D&D game creatures. The monsters created included the bulette and rust monster, and possibly others. This rumor has since been confirmed by the Master himself.
Many years ago I stopped into a KB-Toys on a whim, and there, sittting in a glass container on the counter loaded with rubber spiders and such was a tiny rust monster. I promptly grabbed it, paid for it (it was less than one dollar), and "Lucky the Rust Monster" has been tied to one string of my venerable dice bag ever since. That was sometime in the early 1980s.
I've since seen at least one other picture of a similar plastic critter—identical really, except for a bit more brown color—and confirmed as best possible that I have the same plastic monster EGG grabbed years before. So, without further ado, I present Lucky the Rust Monster...
(Thanks for the EN World folks for inspiring this post!)