vesperto

The cruellest near-pair in the chapter. Vespero at rank 272 is the evening. Vesperto is a bat. One letter, and every learner who has met the first word will read the second as a typo for it. Worse, the sentence usually makes sense either way: bats come out at dusk, so mi vidis vesperton and mi vidis vesperon are both things a person might plausibly be saying, and only one of them is about an animal.

The resemblance is real, which is why the trap works. Latin vespertilio, the bat, was built on vesper, the evening: the creature is named after the hour. Esperanto took the whole Latin word rather than assembling it, so vesperto is a single indivisible root. There is no -t- suffix and vesper + t builds nothing. You cannot decode it; you have to know it.

Where it sits. A vesperto is a besto, rank 373, and specifically a mammal rather than a birdo: the only mammal that truly flies. It roosts in a kaverno, rank 1,634, or under a roof, hanging head downwards with its wings folded round it, which is exactly how the drawing above shows it.