In Greg Svenson's article The First Dungeon Adventure, a participant's account of the very first Blackmoor dungeon delve, the player of the legendary "Great Svenny" recounts the very first monster that the players encountered.
At this point Dave took us into the laundry area of the basement, telling us he wanted to see what we would do. He had us line up in our marching order. Then he turned off the lights saying a sudden wind had blown out our torches. Then we heard some screaming. We generally scattered as best we could. He turned on the lights looked at what we had done and then went back to the other room, telling us that a black blob (like the thing in the classic Japanese horror movie "The Blob" from the 1950’s) had killed one of the NPCs who ran into it. We soon found that our weapons dissolving when we struck it. Then we got some torches relit and found that we could fight it with fire. Eventually we killed it losing a couple more men in the process.I have no idea why Greg remembered The Blob – an American movie starring Steve McQueen – as Japanese, but that was the first monster. It's funny to imagine a bunch of college students (wearing early '70s clothes) running around a dark basement, essentially doing the first LARP, to determine relative position, but these were literally people figuring out how to play these games for the first time.
The fact that the first monster was a blob warms my heart. It wasn't a humanoid or undead, there was no pedigree from mythology or pulp fantasy. Monster Number One was the creature from a bona fide Creature Feature. When look at the time that the guys playing Blackmoor grew up, this makes perfect sense. Science fiction film then didn't mean big budget productions, but B-films with whatever creatures could be cooked up on a shoestring budget. 2001: A Space Odyssey was less than three years old, and Star Wars wouldn't come out for another six years. These were kids who grew up on these movies, of course that's what they imagined.
One twist that I find interesting is that Arneson's blob is killed by fire. This is similar to the OD&D monster Black Pudding (itself something of a culinary joke) and the opposite of the Blob in the eponymous film, which is found to be vulnerable to cold via a CO2 fire extinguisher. Gygax would separate Blob-types into two, with the Black Pudding vulnerable only to fire, and the Gray Ooze harmed by weapons or lightning. The morale effect on a party is quite visible when they find they have used the wrong attack type on a blob, and a torch gutters out harmlessly in a Gray Ooze or the Black Pudding is struck by a lightning bolt and divides in two.
The premier generator for unique blobs every time is the "Goops, Glops and Gobs" section of Geoffrey McKinney's Dungeon of the Unknown. A creature whose vulnerability has not been discovered, but has a movement rate, can easily become the villain of a terrific chase sequence. After all, in The Blob the title creature does ooze under doors and over various surfaces, making getting away from it a difficult problem.
Aside from the importance of the blob itself, I think this shows us the importance of the B-movie as an "Appendix N" for early D&D. Much as Gygax would be influenced by Harryhausen as much as Howard and Vance, this kind of sensibility – the willingness to go in for weird aliens, giant creatures and things of indeterminate origin as "feature" monsters – provides another, deeply old school, alternative to monsters out of the fantasy "canon." And if anybody gives you grief for it, remember - the blob was first.