koboldo

The best small fact in the word is a chemical element. German miners kept finding an ore that looked like silver, refused to yield any, and gave off poisonous fumes when roasted. They decided a Kobold had swapped the good metal for rubbish out of spite and named the stuff after him. When the metal in it was finally isolated in 1735 the name stuck, and that is why the periodic table has kobalto at number 27. The same grudge produced nickel, from Kupfernickel, Old Nick's copper. Two elements named after mine spirits who were, in the miners' view, taking the mickey.

A koboldo is mischievous rather than evil, and that is the register to keep in Esperanto. He hides your keys, sours the milk, knocks in the walls at night, and in the older German telling he will also do your housework if you leave food out for him and stop asking questions. Put him beside the neighbours: a fantomo is a ghost, the returning dead; a feino is a fairy; a trolo is bigger, slower and more dangerous; a gnomo lives underground and guards things. Koboldo is the small, quick, domestic one with a sense of humour.

Built forms, and one useful stretch. Koboldeto is a smaller one and koboldaĉo a nasty one; kobolda is the adjective. Figuratively the word does the same job as English gremlin: when a machine misbehaves for no reason anybody can find, a koboldo has got into it. Esperanto being what it is, you can then say the machine is koboldigita and be understood immediately, which is the sort of thing the language does better than it is given credit for.