Yngvi, Yngvin, Ingwine, Inguin are names that relate to an older theonym Ing and which appears to have been the older name for the god Freyr (originally an epithet, meaning "lord").
Proto-Germanic *Ingwaz was one of the three sons of Mannus and the legendary ancestor of the Ingaevones and is also the reconstructed name of the Elder Futhark ŋ rune.
A torc, the "Ring of Pietroassa", part of a late third- to fourth-century Gothic hoard discovered in Romania, is inscribed in much-damaged runes, one reading of which is gutanī [i(ng)]wi[n] hailag ", "to Ingwi of the Goths. Holy"
The Old Norse name Yngvi is a hypocoristic form of an older and rarer Yngvin (OHG: Inguin, OE: Ingwine), which is derived from the theonym Ing- and means "worshiper or friend of Ing". The theonym would originally have been Proto-Germanic *Inguz, and it appears in Old Norse Ingvifreyr and Ingunarfreyr, as well as in OE fréa inguina, and which mean "Lord of the Inguins", i.e. the god Freyr. The name appears also in Ingvaeones which was an alliance of people surrounding a common cult. Other names that retain the theonym are Inguiomerus/Ingemar and Yngling, the name of an old Scandinavian dynasty.
In Scandinavian mythology, Yngvi, alternatively Yngve, was the progenitor of the Yngling lineage, a legendary dynasty of Swedish kings from whom the earliest historical Norwegian kings in turn claimed to be descended, see also Freyr.
Information on Yngvi varies in different traditions as follows:
* Yngvi is a name of the god Freyr, perhaps intended as Freyr's true name while Frey 'Lord' is his common title. In the Ynglinga saga and in Gesta Danorum, Frey is euhemerized as a king of Sweden. In the Ynglinga saga, Yngvi-Frey reigned in succession to his father Njörd who in turn succeeded Odin. Yngvi-Frey's descendants were the Ynglings.
* In the Íslendingabók Yngvi Tyrkja konungr 'Yngvi king of Turkey' appears as father of Njörd who in turn is the father of Yngvi-Freyr, the ancestor of the Ynglings.
* In the Skjöldunga saga Odin came from Asia and conquered Northern Europe. He gave Sweden to his son Yngvi and Denmark to his son Skjöldr. Since then the kings of Sweden were called Ynglings and those of Denmark Skjöldungs (Scyldings).
* In Historia Norwegiæ, Ingui is the first king of Sweden, and the father of Njord, the father of Freyr: Rex itaque Ingui, quem primum Swethiæ monarchiam rexisse plurimi astruunt, genuit Neorth, qui vero genuit Froy; hos ambos tota illorum posteritas per longa sæcula ut deos venerati sunt. Froyr vero genuit Fiolni, qui in dolio medonis dimersus est,[...].
* In the introduction to Snorri Sturluson's Edda Snorri claims again that Odin reigned in Sweden and relates: "Odin had with him one of his sons called Yngvi, who was king in Sweden after him; and those houses come from him that are named Ynglings." Snorri here does not identify Yngvi and Frey though Frey occasionally appears elsewhere as a son of Odin instead of a son of Njörd. See Sons of Odin.
* In the Skáldskaparmál section of Snorri Sturluson's Edda Snorri brings in the ancient king Halfdan the Old who is the father of nine sons whose names are all words meaning 'king' or 'lord' in Old Norse and nine other sons who are the forefathers of various royal lineages, including "Yngvi, from whom the Ynglings are descended". But rather oddly Snorri immediately follows this with information on what should be four other personages who were not sons of Halfdan but who also fathered dynasties and names the first of these as "Yngvi, from whom the Ynglings are descended". In the related account in the Ættartolur ('Genealogies') attached to Hversu Noregr byggdist, the name Skelfir appears instead of Yngvi in the list of Halfdan's sons. For more details see Scylfing
(The Yngling Saga section of Snorri Sturluson's Heimskringla also introduces a second Yngvi son of Alrek who is a descendant of Yngvi-Frey and who shared the Swedish kingship with his brother Álf. See Yngvi and Alf.)