One of the marks of an epic hero is his leadership and humility. Beowulf traveled with his best men, and was quick to share praise with them. Beowulf, for all his pride, always kept his company around him. The last battle, against the dragon, Beowulf found himself nearly alone, but even then he had loyal Wiglaf by his side.
So, then, the "lone-goer" can't possible be Beowulf. The kenning comes fairly early in the work, and is found in Beowulf's introductory speech to Hrothgar. In that speech, Beowulf says:
Now ween I that he, if he may wield matters,
E’en there in the war-hall the folk of the Geats
Shall eat up unafear’d, as oft he hath done it
With the might of the Hrethmen: no need for thee therefore
My head to be hiding; for me will he have
With gore all bestain’d, if the death of men get me;
He will bear off my bloody corpse minded to taste it;
Unmournfully then will the Lone-goer eat it,
Will blood-mark the moor-ways; for the meat of my body
Naught needest thou henceforth in any wise grieve thee....(Morris, Wyatt, 1910)
Only one creature fits the description of one who "will carry our bloody Flesh to the moors, crunch on our bones And smear torn scraps of our skin on the walls Of his den..." (Raffel, 1999): the monster, Grendel.