The Nonborn King by Julian May (Saga of the Pliocene Exile #3)

On Earth, six million B.C., two species of alien ruled, the graceful humanoid Tanu and their twisted brethren, the Firvulag. Then men from twenty-second century Earth arrived through a one-way time tunnel — and soon the aliens were locked in a battle to the death, for the humans had upset the precarious balance of power that existed between them.
But when the tides of combat had receded, no one group held firm control, though Aiken Drum, man of no woman born, had declared himself the Nonborn King . . . .

Paperback, 1984

Goodreads: 4.19