This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Ophiotaurus (Greek)
In Greek mythology, the Ophiotaurus (Ancient Greek: ??????????) was a creature that was part bull and part serpent. Its only known appearance in an ancient work was in Ovid's Fasti. In this poem, it was the subject of a prophecy which warned that whoever burned the innards of the Ophiotaurus would defeat the gods. Briareus attempted to burn the Ophiotaurus but was foiled by a bird sent by Zeus.
The Ophiotaurus is said to be a creature with the front half of a bull and the back half of a serpent. The only known writing featuring the Ophiotaurus is Fasti by Ovid. Here Ovid describes the types of monstra that are threats to both men and gods. It depicts Briareus, one of the 100-handed Hecatoncheires, killing the Ophiotaurus in an attempt to burn its body, only to be stopped by Zeus.