Dungeons & Dragons: 1st Edition Monster Manual (part 1)
There are several beasties in the Monster Manual that don't fill a believable ecological role. Sure, we can ignore the magically-created creatures and the creatures of myth. We can pretend that thirty separate species of sentient bipeds sharing the same planet is possible. What cannot be faked is the belief that a Rust Monster occupies a sustainable niche in any ecosystem.
Zack: Okay, so a gas spore is a creature that has evolved a reproductive system that relies on fooling hapless adventurers into hacking it open because they think it is one of the most intelligent, rare, and deadly monsters. I see how that could happen.
Steve: Maybe Beholders were hunted nearly to extinction. That would make these spread even more.
Zack: Wait a second, it says in the Beholder description that Beholders are so hateful they almost always attack immediately. These things just float around in caves like loose balloons with a big eyeball sticker on the front. You would have to be a really stupid adventurer to get fooled by this thing.
Steve: What if bats and cave bears were tearing them open and making them spread?
Zack: Okay, fine, but wouldn't those things be afraid of its disguise? It's like a bush growing berries that look like scarecrows.
Steve: Maybe it bumps into stalactites in caves and those rip it open and then its spores shoot out and hit bats.
Zack: Then why does it look like a Beholder?
Steve: God works in mysterious ways.
Zack: Steve, there is no god.
Steve: Tell that to Hextor.