Both of the other answers given here cite 2nd edition AD&D, but your question is tagged 1e.
The 1e Monster Manual describes a class of monsters that are merely traps, conceived for Original D&D or within two years of its debut. These trap-beings include the ear seeker, lurker above, mimic, piercer, rot grub, shrieker, and trapper. As the game was originally conceived and played, none of these was a viable creature in an environment; each provided a surprise encounter, which was then swiftly left behind... 'one-liners', if you will. In the 1e mind-set, nobody worried about their heritage, habits, or any other details -- nor should you, if you play 1e.
Certain aquatic listings -- strangle weed, eye (floating), and water weird -- are arguably in the same class. The "goop group" (gelatinous cube, gray ooze, green slime, ochre jelly) could be treated similarly, but are generally more substantial.
As soon as you start asking why & how regarding these 'one-liners' (c.f. the "Ecology of..." magazine articles), you're moving into the 2e mind-set. :)