The names are different and have little etymological connection. Ongenþeow would in Proto-Norse have been *Anganaþewaz, whereas Egil would have been *Agilaz. The reason why they are thought to have been the same is that they have the same position in the line of Swedish kings and are described as the fathers of Ohthere and grandfathers of Eadgils. As will be shown below, it can be argued that they are based on the same person and the same events, but it should be noted that not every scholar is open to the historicity of the characters in Beowulf, and in the Norse sagas.
The epic tells that the Geats under their new king Hæþcyn captured the Swedish queen, but old king Ongenþeow saved her, at a hill fort called Hrefnesholt, although they lost her gold. Ongentheow killed Hæþcyn, and besieged the Geats at Hrefnesholt. The Geats were, however, rescued by Hygelac, Hæþcyn's brother, who arrived the next day with reinforcements. Having lost the battle, but rescued his queen, Ongenþeow and his warriors returned home.
However, the war was not over. Hygelac, the new king of the Geats, attacked the Swedes. The Geatish warriors Eofor and Wulf fought together against the hoary king Ongenþeow. Wulf hit Ongentheow's head with his sword so that the old king bled over his hair, but the king hit back and wounded Wulf. Then, Eofor retaliated by cutting through the Swedish king's shield and through his helmet, giving Ongentheow a death-blow. Eofor took the Swedish king's helmet, sword and breastplate and carried them to Hygelac. When they came home, Eofor and Wulf were richly awarded, and Eofor was given Hygelac's daughter. Because of this battle, Hygelac is referred to as Ongentheow's slayer.
Ongentheow is also mentioned in passing by the earlier poem Widsith as the king of Sweden:
|Wald Woingum, Wod þyringum,||Wald the Woings, Wod the Thuringians,|
|Sæferð Sycgum, Sweom Ongendþeow,||Saeferth the Sycgs, the Swedes by Ongendtheow,|
|Sceafthere Ymbrum, Sceafa Longbeardum||Sceafthere the Ymbers, Sceafa the Lombards,|
Egil was killed by a bull during the sacrifices at the Temple at Uppsala.
The Historia Norwegiæ presents a Latin summary of Ynglingatal, older than Snorri's quotation:
|Auchun vero genuit Eigil cognomento Vendilcraco, quem proprius servus nomine Tonne regno privavit, et cum domino pedisseqvus VIII civilia bella commisit, in omnibus victoria potitus, in nono tandem devictus occubuit; sed paulo post ipsum regem truculentus taurus confodiens trucidavit. Cui successit in regnum filius suus Ottarus [...]||Aukun's son was Egil Vendelkråke, whose own bondman, Tunne, drove him from his kingdom; and though a mere servant he joined in eight civil combats with his master and won supremacy in all of them, but in a ninth he was finally defeated and killed. Shortly afterwards however the monarch was gored and slaughtered by a ferocious bull. The successor to the throne was his son Ottar, [...]|
If there is any authenticity behind the traditions, the origin of Ynglingatal was most probably a Swedish poem which has not survived (see also Sundquist 2004). In Old Swedish, farra did not mean "bull" but it meant "boar" (cf. English farrow meaning "young pig"). Moreover, in Old Norse Trjóna normally meant a pig's snout (modern Scandinavian tryne). Flæmingr meant "sword" (originally a Flemish sword imported by Vikings).
Moreover, the sword of the snout can hardly refer to the horns of a bull, but it is more natural to interpret it as the tusks of a boar. In English, the lines can be translated as but the giant beast coloured its tusk red on Egil.
In Anglo-Saxon, the name eofor meant "boar" and consequently Ynglingatal could very well relate of Eofor (the boar) killing Egil with kennings for boars. These kennings, sung originally by Swedes, were later misinterpreted by Norwegians and Icelanders as literal expressions due to the different dialectal meanings of farra.
Moreover, according to Schück, the name Tunni which has no meaning in Old Norse should in Proto-Norse have been *Tunþa and derived from *Tunþuz. Consequently, it would have been the same word as the Gothic Tunþus which meant "tooth". This would mean that the name of Egil's enemy, actually meant "tooth" and Tunni and the bull/boar would consequently have been the same enemy, i.e. Eofor.