Diogenes the Cynic

"Homer" is viewed as petty much a purely legendary character. No one knows who or how many authors were actually involved with the composition of the Homeric epics.

Lao Tze, the legendary author of the Tao Te Ching is also viewed as probably being a completely mythical character (the core couplets of the Tao Te Ching arise from different dialects which makes single authoship hard to support. It's more likely that the book was originally a compilation of folk adages which underwent its own process of redaction and editing).