Nowadays, an incubo (nightmare) is a dream full of anguish and fear, but the meaning has changed during the centuries. The italian word, incubo, comes from latin incubare, which means "to sit, to lie on top of someone or something"; a incubo was originally a kind of dwarfish evil creature sitting on the breast of sleepers, making breathing difficult. In english too, the same "shift" has occurred. According to the online etymology dictionary, a nightmare was "an evil female spirit afflicting sleepers with a feeling of suffocation".
Anyway, the original meaning of incubo has been retained in popular culture, and the little creature is the subject of a couple of drawings:
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and a watercolor (alas, the scanner has butchered a bit the colors):
Incidentally, you may want to look at a painting by Johann Heinrich Füssli on the same subject.