This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Onocentaur (Medieval Bestiaries)
The onocentaur is similar to the centaur, but part-human part-donkey. However, unlike a centaur, which is portrayed with four legs, the onocentaur is often portrayed with only two legs where some artwork depicts it with no front legs on the donkey part of it's body. As with many liminal beings, the onocentaur's nature is one of conflict between its human and animal components.
The first known mention was in reign of Ptolemy II Philadelphus by an officer named Pythagoras, as quoted by Claudius Aelianus in De Natura Animalium. Aelian as well uses the term onokentaura for description of the female form. He interpreted the onocentaur as: "its body resembles that of an ass, its colour is ashen but inclines to white beneath the flanks. It has a human chest with teats and a human face surrounded by thick hair.