Perhaps you are thinking of:
* Bellerophon: A figure in Greek mythology who was a hero who slew the Chimera. His name might be associated with letters if you're thinking of Bellerophon's Letter, which is a story about a hero who uses a letter to gain access to a dangerous situation.
* Phonetic Alphabet: This is a system where each symbol represents a specific sound. It could be mistakenly referred to as "bellerophonic" due to the phonetics connection.
* Ancient Writing Systems: If you're looking for the origin of writing systems in general, there are various theories and discoveries regarding the origins of cuneiform, hieroglyphics, and other early writing systems.
Could you please clarify what you mean by "bellerophonic letters"? Perhaps you can provide more context or details about what you are looking for.