Maybe the immortal Starkad in The Boat Of A Million Years is the original of the Starkad of the sagas? The latter was said to have had an extended lifespan granted by Odin.
In SM Stirling's The Protector's War, the bandit Crusher Bailey calls himself Carl Grettir. "Grettir," as in Starkad Grettir, means "Crusher." Mike Havel comments that Bailey is "'...a bandit with some education...'" (p. 284) Well, his creator has more than a little education - and had also read Poul Anderson!
I mentioned five bodies of work linked by the single name, "Starkad." Rearranging these works into a more logical and chronological order gives us:
Anderson's retelling of a saga;
Anderson's historical fiction;
Stirling's alternative history fiction;
Anderson's future history.
That impressive list moves from an ancient past to a remote future with a detour into an alternative timeline created by a second author. Saxo recorded legends about figures like Starkad and both their names are applied to celestial bodies in the future. Anderson's learning linked his heroic fantasies to his hard sf.