Flea

Fleas were animals native to Earth commonly found on dogs, (TV: The Two Doctors) and in "a beggar's robes". (TV: The Masque of Mandragora) According to the Eleventh Doctor, even educated fleas "do it". (TV: Hide)

Behind the scenes
The Doctor's mention of the species in Hide is in reference to Ella Fitzgerald's "":
 * Birds do it, bees do it. Even educated fleas do it. Let's do it. Let's fall in love.