Alaric and Eric - in The Ynglinga Saga

In The Ynglinga Saga

According to the Ynglinga saga, Alaric and Eric were sons and heirs of the previous king Agni by his wife Skjálf. They shared the kingship. They were mighty in both war and sports, but were especially skillful horsmen and vied with one another about their horsemanship and their horses.

One day they rode off from their retinue and did not return. They were found dead with their heads battered but no weapons with them save the bridle bits of their horses. Accordingly it was believed that they had quarreled and come to blows and had slain each other with their bridle bits. They were succeeded by Alaric's sons Yngvi and Alf.

However, in other sources, only Alaric died, and in the piece of Ynglingatal quoted by Snorri Sturluson it is only Alaric who dies explicitly. Eric's death seems to be a misunderstanding on Snorri's part due to an influence from the succeeding kings (see also the other sources below):

Ynglingatal then gives Yngvi and Alf as Alrek's and Eirík's successors.

The Historia Norwegiæ presents a Latin summary of Ynglingatal, older than Snorri's quotation:

Hogna is an error for Agne. Unlike Ynglingatal, Historia Norwegiæ gives Dagr as Alrekr's predecessor. Instead Alrekr precedes Agne and Agne is succeeded by Yngvi (incorrectly called Ingialdr). The even earlier source Íslendingabók cites the line of descent in Ynglingatal and it gives the same line of succession as Historia Norwegiæ: xi Dagr. xii Alrekr. xiii Agni. xiiii Yngvi.