Egyptian Plover or Crocodile Bird 
cleaning Nile Crocodile's teeth

This is a digitally constructed image to illustrate the popular myth attributed to Herodotus, 5th Century BC. Two eminent ornithologists (Richford and Mead, 2003) have supported this story anecdotally, but the behavior has never been authenticated.  There are no actual photos of the behavior, but there have been many illustrations.