Belzoidian flea

From Memory Alpha, the free Star Trek reference

Belzoidian flea is a mortal lifeform of unknown origin.

In 2366, when Q was stripped of his omnipotence and immortality, he thought of taking the form of a Belzoidian flea before deciding on taking the form of Human, as part of his punishment by the Q Continuum for his irresponsibility. (TNG: "Deja Q")