I think I acquired the book as a hand-me-down from my DM friend when he got the Advanced books. I read it lovingly and got to know it, even though all our play went into 1e.
There were several monsters that never crossed over into 1e, which helped give the two systems, which seem very close rules-wise, different flavors for me. So, I dug out my Moldvay and perused it again. Here are some of the monsters unique to the red book:
- Ape, White
- Cave Locust
- Living Statue (Crystal, Iron, Rock)
- Rock Baboon
Now this is probably old news to many of you. But it's interesting for me to look at the different flavors as someone who spent my gaming career focused on 1e. The cave locust seemed interesting to me way back then, I may have even made a little dungeon with them placed in it. I think I liked them because they could be a logical fit to the environment, Gygaxian naturalism, I suppose. But now looking back at them I realized they were a Deadly Distraction! Look at some highlights from their description:
- They are camouflaged in their stone environments and may look like a statue
- They are nervous and will flee rather than fight
- It can accidentally jump into characters, injuring them
- Frightened or attacked, they start shrieking, drawing wandering monsters
- Cornered, it will spit brown yuck that makes you unable to do anything for a turn and causes people to get sick until you wash it off.
But think about it, this one monster is practically a primer in old-school encounters: Be cautious, "Hey look at this statue over here . . . "; Fighting everything is a bad strategy, "Now it started shrieking, it's shrieking!"; Umm, yeah, fighting everything is a bad strategy, "It spit on me . . . oh, the smell . . . blarrrgh."
Who designed this creature? Anyone know? I'd like to congratulate that person on a finely designed old-school monster. And I hadn't planned it when I started the post, but I can't resist, my Swords & Wizardy conversion homage:
Armor Class: 4 
Hit Dice: 1d6
Attacks: Bump (1d4) or Spit
Special: Spittle causes nausea
Move: 6/18 (flying)
Challenge Level/XP: 1/15
These skittish herbivores the size of a dog, are a mottled-grey that blends into cavern walls. If seen, they appear to be a large statue of a locust. If bothered, their first inclination is to flee, which entails leaping with their powerful hind legs. Unfortunate adventurers in the path of such a jump will not soon forget it.
Cave locusts unable to flee unhindered begin emitting a piercing, shriek-like noise that may draw wandering monsters. As a last resort, a cornered cave locust will spew brown spittle on attackers. This sticky mess is so noisome as to cause anyone smelling it to become nauseous enough to hinder their ability to move and fight.